Solid Electrolyte, Method For Producing Solid Electrolyte, And Composite Body

ABSTRACT

A solid electrolyte according to the present disclosure is represented by the following compositional formula (1).Li7(La3-xBix)Zr2O12  (1)In the formula (1), x satisfies 0.05&lt;x&lt;0.40.

The present application is based on, and claims priority from JP Application Serial Number 2020-040062, filed on Mar. 9, 2020, the disclosure of which is hereby incorporated by reference herein in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a solid electrolyte, a method for producing a solid electrolyte, and a composite body.

2. Related Art

As a power supply for many electrical apparatuses such as portable information apparatuses, a lithium battery (including a primary battery and a secondary battery) has been used. Above all, as a lithium battery that achieves both a high energy density and safety, an all-solid-state lithium battery using a solid electrolyte for lithium conduction between positive and negative electrodes has been proposed (see, for example, JP-A-2009-215130 (Patent Document 1).

A solid electrolyte can conduct lithium ions without using an organic electrolyte solution, and leakage of an electrolyte solution or volatilization of an electrolyte solution due to drive heat generation, or the like does not occur, and therefore, it has been attracting attention as a highly safe material.

As a solid electrolyte to be used in such an all-solid-state lithium battery, an oxide-based solid electrolyte having a high lithium ion conduction property, an excellent insulation property, and high chemical stability has been widely known. As such an oxide, a lithium lanthanum zirconate-based material has a remarkably high lithium ion conductivity, and application thereof to a battery has been expected.

When such a solid electrolyte is solid electrolyte particles in a particulate shape, the solid electrolyte is often molded according to a desired shape by compression molding. However, the solid electrolyte particles are very hard, and therefore, in the obtained molded product, the contact between the solid electrolyte particles is not sufficient and the grain boundary resistance becomes high, and thus, the lithium ion conductivity is likely to be low.

As a method for reducing the grain boundary resistance, a method of fusing solid electrolyte particles by sintering at a high temperature of 1000° C. or higher after compression molding the particles is known. However, in such a method, the formulation is likely to be changed due to the high temperature, and it is difficult to produce a solid electrolyte molded body having desired physical properties.

Therefore, there has been an attempt to form a material suitable for sintering at a low temperature by substituting some elements in lithium lanthanum zirconate.

However, a solid electrolyte capable of obtaining a solid electrolyte molded body having a sufficiently low grain boundary resistance at a sufficiently low firing temperature has not yet been obtained.

SUMMARY

The present disclosure has been made for solving the above problem and can be realized as the following application examples.

A solid electrolyte according to an application example of the present disclosure is represented by the following compositional formula (1).

Li₇(La_(3-x)Bi_(x))Zr₂O₁₂  (1)

In the formula (1), x satisfies 0.05<x<0.40.

Further, a method for producing a solid electrolyte according to an application example of the present disclosure includes: a mixing step of mixing multiple types of raw materials containing metal elements included in the following compositional formula (1), thereby obtaining a mixture; a first heating step of subjecting the mixture to a first heating treatment thereby forming a calcined body; and a second heating step of subjecting the calcined body to a second heating treatment thereby forming a crystalline solid electrolyte represented by the following compositional formula (1).

Li₇(La_(3-x)Bi_(x))Zr₂O₁₂  (1)

In the formula (1), x satisfies 0.05<x<0.40.

Further, a composite body according to an application example of the present disclosure includes: an active material; and the solid electrolyte according to the present disclosure that coats a part of a surface of the active material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as a secondary battery of a first embodiment.

FIG. 2 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as a secondary battery of a second embodiment.

FIG. 3 is a schematic cross-sectional view schematically showing a structure of the lithium-ion battery as the secondary battery of the second embodiment.

FIG. 4 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as a secondary battery of a third embodiment.

FIG. 5 is a schematic cross-sectional view schematically showing a structure of the lithium-ion battery as the secondary battery of the third embodiment.

FIG. 6 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as a secondary battery of a fourth embodiment.

FIG. 7 is a schematic cross-sectional view schematically showing a structure of the lithium-ion battery as the secondary battery of the fourth embodiment.

FIG. 8 is a flowchart showing a method for producing the lithium-ion battery as the secondary battery of the first embodiment.

FIG. 9 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the first embodiment.

FIG. 10 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the first embodiment.

FIG. 11 is a schematic cross-sectional view schematically showing another method for forming a solid electrolyte layer.

FIG. 12 is a flowchart showing a method for producing the lithium-ion battery as the secondary battery of the second embodiment.

FIG. 13 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the second embodiment.

FIG. 14 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the second embodiment.

FIG. 15 is a flowchart showing a method for producing the lithium-ion battery as the secondary battery of the third embodiment.

FIG. 16 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the third embodiment.

FIG. 17 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the third embodiment.

FIG. 18 is a flowchart showing a method for producing the lithium-ion battery as the secondary battery of the fourth embodiment.

FIG. 19 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the fourth embodiment.

FIG. 20 is a view schematically showing a sweep potential-response current graph obtained by CV measurement.

FIG. 21 is a view showing the Raman scattering spectra of solid electrolytes of Example 1 and Comparative Example 2.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, preferred embodiments of the present disclosure will be described in detail.

[1] Solid Electrolyte

First, a solid electrolyte according to the present disclosure will be described.

The solid electrolyte according to the present disclosure is represented by the following compositional formula (1).

Li₇(La_(3-x)Bi_(x))Zr₂O₁₂  (1)

In the formula (1), x satisfies 0.05<x<0.40.

By satisfying such conditions, a solid electrolyte that has an excellent bulk lithium ion conductivity, and is also capable of obtaining a solid electrolyte molded body having a sufficiently low grain boundary resistance at a sufficiently low firing temperature can be provided. Further, a solid electrolyte in the related art has a problem that when, for example, the solid electrolyte is co-fired with an active material such as lithium cobalt oxide, mutual diffusion of elements in the respective materials occurs to decrease the lithium ion conductivity, however, in the solid electrolyte according to the present disclosure, even when the solid electrolyte is co-fired with an active material such as lithium cobalt oxide, the occurrence of the problem that mutual diffusion of elements in the respective materials occurs to decrease the lithium ion conductivity can be effectively suppressed.

The reason why such an excellent effect is obtained is considered as follows.

That is, it is considered that due to the following fact:

(1) Bi and La have a given difference in atomic radius, that is, while La has an atomic radius of 194 pm, Bi has an atomic radius of 150 pm,

(2) the melting point of Bi in the form of a metal simple substance is significantly lower than that of La, that is, while the melting point of metal La is 920° C., the melting point of metal Bi is 271.4° C.,

or the like, the sintering temperature of a solid electrolyte is lowered, or the crystal phase transition temperature of a solid electrolyte is lowered, so that an excellent effect as described above is obtained.

On the other hand, when the conditions as described above are not satisfied, an excellent effect as described above is not obtained.

For example, when a solid electrolyte does not contain Bi, it becomes difficult to make the bulk lithium ion conductivity sufficiently excellent, and also it becomes difficult to obtain a solid electrolyte molded body having a sufficiently low grain boundary resistance at a sufficiently low firing temperature.

Further, even if a solid electrolyte is a lithium lanthanum zirconate-based material containing Bi, when the content of Bi in the solid electrolyte is too low, in other words, when the value of the x is too small, it becomes difficult to make the bulk lithium ion conductivity sufficiently excellent, and also it becomes difficult to obtain a solid electrolyte molded body having a sufficiently low grain boundary resistance at a sufficiently low firing temperature.

Further, even if a solid electrolyte is a lithium lanthanum zirconate-based material containing Bi, when the content of Bi in the solid electrolyte is too high, in other words, when the value of the x is too large, it becomes difficult to make the bulk lithium ion conductivity sufficiently excellent, and also it becomes difficult to obtain a solid electrolyte molded body having a sufficiently low grain boundary resistance at a sufficiently low firing temperature.

As described above, in the compositional formula (1), x need only satisfy the condition: 0.05<x<0.40, but preferably satisfies the condition: 0.07≤x≤0.38, more preferably satisfies the condition: 0.10≤x≤0.35, and further more preferably satisfies the condition: 0.15≤x≤0.30.

According to this, the above-mentioned effect is more remarkably exhibited.

In the solid electrolyte, Li is mainly present at C site and an interstitial position in a garnet-type lithium ion conductor Li₇La₃Zr₂O₁₂ that is a basic structure, and contributes to the lithium ion conduction property.

In the solid electrolyte, La mainly constitutes a garnet-type lithium ion conductor Li₇La₃Zr₂O₁₂ that is a basic structure, and occupies A site as La³⁺.

In the solid electrolyte, Bi mainly lowers the tetragonal-cubic transition temperature and the melting point as compared with a case where Bi is not contained, and promotes the crystal bulk growth and contributes to the reduction of grain boundaries.

In the solid electrolyte, Zr mainly constitutes a garnet-type lithium ion conductor Li₇La₃Zr₂O₁₂ that is a basic structure, and occupies B site as Zr⁴⁺.

The solid electrolyte according to the present disclosure may contain another element in addition to the elements constituting the compositional formula (1), that is, an element other than Li, La, Bi, Zr, and O as long as the amount thereof is a trace amount. The another element may be one type or two or more types.

The content of the another element contained in the solid electrolyte according to the present disclosure is preferably 100 ppm or less, and more preferably 50 ppm or less.

When two or more types of elements are contained as the another element, the sum of the contents of these elements shall be adopted as the content of the another element.

The crystal phase of the solid electrolyte according to the present disclosure is generally a cubic garnet-type crystal.

The solid electrolyte according to the present disclosure may be, for example, used by itself or in combination with another component. More specifically, the solid electrolyte according to the present disclosure may be, for example, used as a substance that constitutes a solid electrolyte layer as described later in a battery by itself, or may be a substance that constitutes a solid electrolyte layer in a mixed state with another solid electrolyte. In addition, for example, the solid electrolyte according to the present disclosure may be a substance that constitutes a positive electrode layer in a mixed state with a positive electrode active material, or may be a substance that constitutes a negative electrode layer in a mixed state with a negative electrode active material.

[2] Method for Producing Solid Electrolyte

Next, a method for producing a solid electrolyte according to the present disclosure will be described.

The method for producing a solid electrolyte according to the present disclosure includes a mixing step of mixing multiple types of raw materials containing metal elements included in the compositional formula (1), thereby obtaining a mixture, a first heating step of subjecting the mixture to a first heating treatment thereby forming a calcined body, and a second heating step of subjecting the calcined body to a second heating treatment thereby forming a crystalline solid electrolyte represented by the compositional formula (1).

According to this, a method for producing a solid electrolyte that has an excellent bulk lithium ion conductivity, and is also capable of obtaining a solid electrolyte molded body having a sufficiently low grain boundary resistance at a sufficiently low firing temperature can be provided. Further, a solid electrolyte in the related art has a problem that when, for example, the solid electrolyte is co-fired with an active material such as lithium cobalt oxide, mutual diffusion of elements in the respective materials occurs to decrease the lithium ion conductivity, however, in the method for producing a solid electrolyte according to the present disclosure, even when it is applied to co-firing with an active material such as lithium cobalt oxide, the occurrence of the problem that mutual diffusion of elements in the respective materials occurs to decrease the lithium ion conductivity can be effectively suppressed.

[2-1] Mixing Step

In the mixing step, a mixture is obtained by mixing multiple types of raw materials containing metal elements included in the compositional formula (1).

In this step, two or more types of metal elements among the metal elements included in the compositional formula (1) need only be contained in the mixture obtained by mixing multiple types of raw materials as a whole.

Further, at least one type of the multiple types of raw materials to be used in this step may be an oxoacid compound containing an oxoanion together with a metal ion.

According to this, by a heat treatment at a relatively low temperature for a relatively short time, a solid electrolyte having a desired property can be stably formed. More specifically, by using an oxoacid compound in this step, a calcined body can be obtained as a material containing an oxide that is different from a solid electrolyte to be finally obtained and an oxoacid compound in the later step. As a result, the melting point of the oxide is lowered, and a close contact interface with an adherend can be formed while promoting the crystal growth in a firing treatment that is a heat treatment at a relatively low temperature for a relatively short time.

The oxoanion constituting the oxoacid compound does not contain a metal element, and for example, a halogen oxoacid, a borate ion, a carbonate ion, an orthocarbonate ion, a carboxylate ion, a silicate ion, a nitrite ion, a nitrate ion, a phosphite ion, a phosphate ion, an arsenate ion, a sulfite ion, a sulfate ion, a sulfonate ion, a sulfinate ion, and the like are exemplified. As the halogen oxoacid, for example, a hypochlorous ion, a chlorite ion, a chlorate ion, a perchlorate ion, a hypobromite ion, a bromite ion, a bromate ion, a perbromate ion, a hypoiodite ion, an iodite ion, an iodate ion, a periodate ion, and the like are exemplified. Above all, the oxoacid compound preferably contains at least one of a nitrate ion and a sulfate ion as the oxoanion, and more preferably contains a nitrate ion.

According to this, the melting point of the metal oxide contained in the calcined body obtained in the first heating step that will be described in detail later is more favorably lowered, and the crystal growth of a lithium-containing composite oxide can be more effectively promoted. As a result, even when the second heating step that will be described in detail later is performed at a lower temperature for a shorter time, a solid electrolyte having a particularly excellent ion conduction property can be favorably obtained. In the following description, a metal oxide contained in the calcined body obtained in the first heating step is also referred to as “precursor oxide”.

As the raw materials containing metal elements, for example, a simple substance metal or an alloy may be used, or a compound in which a metal element contained in a molecule is only one type may be used, or a compound containing multiple types of metal elements in a molecule may be used.

As a lithium compound that is a raw material containing Li, for example, a lithium metal salt, a lithium alkoxide, and the like are exemplified, and it is possible to use one type or two or more types in combination among these. Examples of the lithium metal salt include lithium chloride, lithium nitrate, lithium sulfate, lithium acetate, lithium hydroxide, lithium carbonate, and (2,4-pentanedionato)lithium. Examples of the lithium alkoxide include lithium methoxide, lithium ethoxide, lithium propoxide, lithium isopropoxide, lithium butoxide, lithium isobutoxide, lithium sec-butoxide, lithium tert-butoxide, and dipivaloylmethanato lithium. Above all, the lithium compound is preferably one type or two or more types selected from the group consisting of lithium nitrate, lithium sulfate, and (2,4-pentanedionato)lithium. As the raw material containing Li, a hydrate may be used.

As a lanthanum compound that is a raw material containing La, for example, a lanthanum metal salt, a lanthanum alkoxide, lanthanum hydroxide, and the like are exemplified, and it is possible to use one type or two or more types in combination among these. Examples of the lanthanum metal salt include lanthanum chloride, lanthanum nitrate, lanthanum sulfate, lanthanum acetate, and tris(2,4-pentanedionato) lanthanum. Examples of the lanthanum alkoxide include lanthanum trimethoxide, lanthanum triethoxide, lanthanum tripropoxide, lanthanum triisopropoxide, lanthanum tributoxide, lanthanum triisobutoxide, lanthanum tri-sec-butoxide, lanthanum tri-tert-butoxide, and dipivaloylmethanato lanthanum. Above all, the lanthanum compound is preferably at least one type selected from the group consisting of lanthanum nitrate, tris(2,4-pentanedionato)lanthanum, and lanthanum hydroxide. As the raw material containing La, a hydrate may be used.

As a bismuth compound that is a raw material containing Bi, for example, a bismuth metal salt, a bismuth alkoxide, bismuth hydroxide, and the like are exemplified, and it is possible to use one type or two or more types in combination among these. Examples of the bismuth metal salt include bismuth chloride, bismuth nitrate, bismuth sulfate, bismuth acetate, and bismuth 2-ethylhexanoate. Examples of the bismuth alkoxide include bismuth triisopropoxide, bismuth triethoxide, bismuth tri-tert-amiloxide, and bismuth tris(dipivaloylmethanate). Above all, the bismuth compound is preferably at least one type selected from the group consisting of bismuth nitrate, bismuth 2-ethylhexanoate, bismuth triisopropoxide, and bismuth triethoxide. As the raw material containing Bi, a hydrate may be used.

As a zirconium compound that is a raw material containing Zr, for example, a zirconium metal salt, a zirconium alkoxide, and the like are exemplified, and it is possible to use one type or two or more types in combination among these. Examples of the zirconium metal salt include zirconium chloride, zirconium oxychloride, zirconium oxynitrate, zirconium oxysulfate, zirconium oxyacetate, and zirconium acetate. Examples of the zirconium alkoxide include zirconium tetramethoxide, zirconium tetraethoxide, zirconium tetrapropoxide, zirconium tetraisopropoxide, zirconium tetra-n-butoxide, zirconium tetraisobutoxide, zirconium tetra-sec-butoxide, zirconium tetra-tert-butoxide, and dipivaloylmethanato zirconium. Above all, as the zirconium compound, zirconium tetra-n-butoxide is preferred. As the raw material containing Zr, a hydrate may be used.

In the preparation of the mixture, a solvent may be used.

According to this, the multiple types of raw materials containing metal elements included in the compositional formula (1) can be more favorably mixed.

The solvent is not particularly limited, and for example, various types of organic solvents can be used, however, more specifically, for example, an alcohol, a glycol, a ketone, an ester, an ether, an organic acid, an aromatic, an amide, and the like are exemplified, and one type or a mixed solvent that is a combination of two or more types selected from these can be used. Examples of the alcohol include methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, allyl alcohol, and 2-n-butoxyethanol. Examples of the glycol include ethylene glycol, propylene glycol, butylene glycol, hexylene glycol, pentanediol, hexanediol, heptanediol, and dipropylene glycol. Examples of the ketone include dimethyl ketone, methyl ethyl ketone, methyl propyl ketone, and methyl isobutyl ketone. Examples of the ester include methyl formate, ethyl formate, methyl acetate, and methyl acetoacetate. Examples of the ether include diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol dimethyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and dipropylene glycol monomethyl ether. Examples of the organic acid include formic acid, acetic acid, 2-ethylbutyric acid, and propionic acid. Examples of the aromatic include toluene, o-xylene, and p-xylene. Examples of the amide include formamide, N,N-dimethylformamide, N,N-diethylformamide, dimethylacetamide, and N-methylpyrrolidone. Above all, the solvent is preferably at least one of 2-n-butoxyethanol and propionic acid.

When a solvent is used in the preparation of the mixture, the solvent may be at least partially removed prior to the first heating step that will be described in detail later.

The removal of the solvent prior to the first heating step can be performed by, for example, heating the mixture, placing the mixture in a reduced pressure atmosphere, or placing the mixture under normal temperature and normal pressure. By at least partially removing the solvent, the mixture can be favorably gelled. Note that the “normal temperature and normal pressure” as used herein refers to 25° C. and 1 atm.

Hereinafter, when the removal of the solvent is performed by a heating treatment, the heating treatment is also referred to as “preheating treatment”.

The conditions of the preheating treatment depend on the boiling point or the vapor pressure of the solvent or the like, but the heating temperature in the preheating treatment is preferably 50° C. or higher and 250° C. or lower, more preferably 60° C. or higher and 230° C. or lower, and further more preferably 80° C. or higher and 200° C. or lower. During the preheating treatment, the heating temperature may be changed. For example, the preheating treatment may include a first stage in which a heat treatment is performed while maintaining a relatively low temperature, and a second stage in which the temperature is raised after the first stage, and a heat treatment at a relatively high temperature is performed. In such a case, it is preferred that the highest temperature in the preheating treatment falls within the above-mentioned range.

Further, the heating time in the preheating treatment is preferably 10 minutes or more and 180 minutes or less, more preferably 20 minutes or more and 120 minutes or less, and further more preferably 30 minutes or more and 60 minutes or less.

The preheating treatment may be performed in any atmosphere, and may be performed in an oxidizing atmosphere such as in the air or in an oxygen gas atmosphere, or may be performed in a non-oxidizing atmosphere of an inert gas such as nitrogen gas, helium gas, or argon gas, or the like. Further, the preheating treatment may be performed under reduced pressure or vacuum, or under pressure.

Further, during the preheating treatment, the atmosphere may be maintained under substantially the same conditions, or may be changed to different conditions. For example, the preheating treatment may include a first stage in which a heat treatment is performed in a normal pressure environment and a second stage in which a heat treatment is performed in a reduced pressure environment after the first stage.

[2-2] First Heating Step

In the first heating step, the mixture obtained in the mixing step, for example, the gelled mixture is subjected to a first heating treatment, thereby forming a calcined body.

In particular, when an oxoacid compound is used for part of the raw materials, a calcined body containing a precursor oxide that is an oxide different from the solid electrolyte to be finally obtained, and the oxoacid compound is obtained.

The heating temperature in the first heating step is not particularly limited, but is preferably 500° C. or higher and 650° C. or lower, more preferably 510° C. or higher and 650° C. or lower, and further more preferably 520° C. or higher and 600° C. or lower.

According to this, undesirable vaporization of metal elements to constitute the solid electrolyte to be finally obtained, particularly vaporization of Li that easily vaporizes among the metal materials, or the like can be more effectively prevented, and the formulation of the solid electrolyte to be finally obtained can be more strictly controlled, and also the solid electrolyte can be more efficiently produced.

During the first heating step, the heating temperature may be changed. For example, the first heating step may include a first stage in which a heat treatment is performed while maintaining a relatively low temperature, and a second stage in which the temperature is raised after the first stage, and a heat treatment at a relatively high temperature is performed. In such a case, it is preferred that the highest temperature in the first heating step falls within the above-mentioned range.

Further, the heating time in the first heating step, particularly the heating time when the heating temperature is 500° C. or higher and 650° C. or lower is preferably 5 minutes or more and 180 minutes or less, more preferably 10 minutes or more and 120 minutes or less, and further more preferably 15 minutes or more and 90 minutes or less.

The first heating step may be performed in any atmosphere, and may be performed in an oxidizing atmosphere such as in the air or in an oxygen gas atmosphere, or may be performed in a non-oxidizing atmosphere of an inert gas such as nitrogen gas, helium gas, or argon gas, or the like. Further, the first heating step may be performed under reduced pressure or vacuum, or under pressure. In particular, the first heating step is preferably performed in an oxidizing atmosphere.

Further, during the first heating step, the atmosphere may be maintained under substantially the same conditions, or may be changed to different conditions. For example, the first heating step may include a first stage in which a heat treatment is performed in an inert gas atmosphere and a second stage in which a heat treatment is performed in an oxidizing atmosphere after the first stage.

The calcined body obtained as described above generally contains a precursor oxide having a crystal phase which is different from that of the solid electrolyte to be finally obtained, that is, the solid electrolyte represented by the compositional formula (1) at normal temperature and normal pressure. In this specification, the “different” in terms of crystal phase is a broad concept not only including that the type of crystal phase is not the same, but also including that even if the type is the same, at least one lattice constant is different, or the like.

As the crystal phase of the precursor oxide, for example, cubic crystals such as a pyrochlore-type crystal, a perovskite structure, a rock salt-type structure, a diamond structure, a fluorite-type structure, and a spinel-type structure, orthorhombic crystals such as a ramsdellite type, a trigonal crystal such as a corundum type, and the like are exemplified, however, the crystal phase is preferably a pyrochlore-type crystal.

According to this, even when the conditions in the second heating step described later are set to a lower temperature and a shorter time, a solid electrolyte having a particularly excellent ion conduction property can be favorably obtained.

The crystal grain diameter of the precursor oxide is not particularly limited, but is preferably 10 nm or more and 200 nm or less, more preferably 15 nm or more and 180 nm or less, and further more preferably 20 nm or more and 160 nm or less.

According to this, due to a so-called Gibbs-Thomson effect that is a phenomenon of lowering the melting point with an increase in surface energy, the melting temperature of the precursor oxide or the firing temperature in the second heating step can be further lowered. Further, this is also advantageous to the improvement of joining of the solid electrolyte to be produced using the method according to the present disclosure to a heterogeneous material or the reduction of the defect density.

The precursor oxide is preferably constituted by a substantially single crystal phase.

According to this, the precursor oxide undergoes crystal phase transition substantially once when producing the solid electrolyte using the method according to the present disclosure, that is, when generating a high-temperature crystal phase, and therefore, segregation of elements accompanying the crystal phase transition or generation of a contaminant crystal by thermal decomposition is suppressed, so that various properties of the solid electrolyte to be produced are further improved.

In a case where only one exothermic peak is observed within a range of 300° C. or higher and 1,000° C. or lower when measurement is performed by TG-DTA at a temperature raising rate of 10° C./min for the calcined body obtained in the first heating step, it can be determined that “it is constituted by a substantially single crystal phase”.

The formulation of the precursor oxide is not particularly limited, however, the precursor oxide is preferably a composite oxide. In particular, the precursor oxide is preferably a composite oxide containing Li and La.

According to this, even when a heat treatment in the second heating step described later is performed at a lower temperature for a shorter time, a solid electrolyte having a particularly excellent ion conduction property can be favorably obtained. In addition, for example, in an all-solid-state secondary battery, the adhesion of the solid electrolyte to be formed to a positive electrode active material or a negative electrode active material can be made more excellent, and a composite material can be formed so as to have a more favorable close contact interface, and thus, the properties and reliability of the all-solid-state secondary battery can be made more excellent.

Further, in the calcined body obtained as described above, generally, almost all the solvent used in the production process is removed, however, a portion of the solvent may remain. However, the content of the solvent in the calcined body is preferably 1.0 mass % or less, and more preferably 0.1 mass % or less.

[2-3] Second Heating Step

In the second heating step, the calcined body obtained in the first heating step is subjected to a second heating treatment, thereby forming a crystalline solid electrolyte represented by the compositional formula (1).

In particular, when the calcined body obtained in the first heating step contains an oxoacid compound, the melting point of the precursor oxide is favorably lowered, and the crystal growth of a lithium-containing composite oxide can be promoted, and by a heat treatment at a relatively low temperature for a relatively short time, a solid electrolyte having a desired property can be stably formed. In addition, the adhesion between the solid electrolyte to be formed and an adherend can be made favorable.

The second heating step may be performed after mixing another component in the calcined body described above.

For example, the second heating step may be performed for a mixture of the calcined body with an oxoacid compound.

Even in such a case, the same effect as described above is obtained.

Here, as a specific example of the oxoacid compound that can be mixed with the calcined body, the oxoacid compound contained in the metal compound exemplified as the raw material of the mixture described above, or the like is exemplified.

In the second heating step, the calcined body may be subjected to the heating step in a state of being mixed with an active material such as a positive electrode active material or a negative electrode active material.

According to this, an electrode such as a positive electrode or a negative electrode can be favorably produced in a state where the solid electrolyte is contained together with the active material. The positive electrode active material and the negative electrode active material will be described in detail later.

A composition to be subjected to the second heating step contains multiple types of metal elements as the constituent elements of the solid electrolyte as a whole, and generally, the ratio of the contents thereof corresponds to the content ratio of each metal element in the formulation of the target solid electrolyte, that is, the compositional formula (1).

When the composition to be subjected to this step contains an oxoacid compound, the content of the oxoacid compound in the composition is not particularly limited, but is preferably 0.1 mass % or more and 20 mass % or less, more preferably 1.5 mass % or more and 15 mass % or less, and further more preferably 2.0 mass % or more and 10 mass % or less.

According to this, a heat treatment in the second heating step can be favorably performed at a lower temperature for a shorter time while more reliably preventing the oxoacid compound from undesirably remaining in the solid electrolyte to be finally obtained, and the ion conduction property of the solid electrolyte to be obtained can be made particularly excellent.

The content of the precursor oxide in the composition to be subjected to this step is not particularly limited, but is preferably 35 mass % or more and 85 mass % or less, and more preferably 45 mass % or more and 90 mass % or less.

When the content of the precursor oxide in the composition to be subjected to this step is represented by XP [mass %] and the content of the oxoacid compound in the composition to be subjected to this step is represented by XO [mass %], it is preferred to satisfy a relationship: 0.013≤XO/XP≤0.58, it is more preferred to satisfy a relationship: 0.023≤XO/XP≤0.34, and it is further more preferred to satisfy a relationship: 0.03≤XO/XP≤0.19.

According to this, a heat treatment in the second heating step can be favorably performed at a lower temperature for a shorter time while more reliably preventing the oxoacid compound from undesirably remaining in the solid electrolyte to be finally obtained, and the ion conduction property of the solid electrolyte to be obtained can be made particularly excellent.

The heating temperature in the second heating step is not particularly limited, but is generally a higher temperature than the heating temperature in the first heating step, and is preferably 800° C. or higher and 1000° C. or lower, more preferably 810° C. or higher and 980° C. or lower, and further more preferably 820° C. or higher and 950° C. or lower.

According to this, by a heat treatment at a relatively low temperature for a relatively short time, a solid electrolyte having a desired property can be stably formed. Further, since a solid electrolyte can be produced by a heat treatment at a relatively low temperature for a relatively short time, for example, the productivity of the solid electrolyte or an all-solid-state battery including the solid electrolyte can be made more excellent, and also from the viewpoint of energy saving, such a heat treatment is preferred.

During the second heating step, the heating temperature may be changed. For example, the second heating step may include a first stage in which a heat treatment is performed while maintaining a relatively low temperature, and a second stage in which the temperature is raised after the first stage, and a heat treatment at a relatively high temperature is performed. In such a case, it is preferred that the highest temperature in the second heating step falls within the above-mentioned range.

The heating time in the second heating step, particularly the heating time when the heating temperature is 800° C. or higher and 1000° C. or lower is, but not particularly limited to, preferably 5 minutes or more and 600 minutes or less, more preferably 10 minutes or more and 540 minutes or less, and further more preferably 15 minutes or more and 500 minutes or less.

According to this, by a heat treatment at a relatively low temperature for a relatively short time, a solid electrolyte having a desired property can be stably formed. Further, since the solid electrolyte can be produced by a heat treatment at a relatively low temperature for a relatively short time, for example, the productivity of the solid electrolyte or an all-solid-state battery including the solid electrolyte can be made more excellent, and also from the viewpoint of energy saving, such a heat treatment is preferred.

The second heating step may be performed in any atmosphere, and may be performed in an oxidizing atmosphere such as in the air or in an oxygen gas atmosphere, or may be performed in a non-oxidizing atmosphere of an inert gas such as nitrogen gas, helium gas, or argon gas, or the like. Further, the heating step may be performed under reduced pressure or vacuum, or under pressure. In particular, the second heating step is preferably performed in an oxidizing atmosphere.

Further, during the second heating step, the atmosphere may be maintained under substantially the same conditions, or may be changed to different conditions.

Even when the oxoacid compound is used as a raw material, the solid electrolyte obtained as described above generally does not substantially contain the oxoacid compound. More specifically, the content of the oxoacid compound in the solid electrolyte to be obtained is generally 100 ppm or less, and particularly, it is preferably 50 ppm or less, and more preferably 10 ppm or less.

According to this, the content of an undesirable impurity in the solid electrolyte can be suppressed, and the properties and reliability of the solid electrolyte can be made more excellent.

[3] Composite Body

Next, a composite body according to the present disclosure will be described.

The composite body according to the present disclosure includes an active material and the solid electrolyte according to the present disclosure that coats a part of a surface of the active material.

According to this, a composite body having a sufficiently low grain boundary resistance between the active material and the solid electrolyte can be provided. Such a composite body can be favorably applied to a positive electrode composite material or a negative electrode composite material of a secondary battery as described later. As a result, the properties and reliability of the secondary battery as a whole can be made excellent.

Examples of the active material constituting the composite body according to the present disclosure include a positive electrode active material and a negative electrode active material.

As the positive electrode active material, for example, a lithium composite oxide which contains at least Li and is constituted by any one or more types of elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, and Cu, or the like can be used. Examples of such a composite oxide include LiCoO₂, LiNiO₂, LiMn₂O₄, Li₂Mn₂O₃, LiCr_(0.5)Mn_(0.5)O₂, LiFePO₄, Li₂FeP₂O₇, LiMnPO₄, LiFeBO₃, Li₃V₂ (PO₄)₃, Li₂CuO₂, Li₂FeSiO₄, and Li₂MnSiO₄. Further, as the positive electrode active material, for example, a fluoride such as LiFeF₃, a boride complex compound such as LiBH₄ or Li₄BN₃H₁₀, an iodine complex compound such as a polyvinylpyridine-iodine complex, a nonmetallic compound such as sulfur, or the like can also be used.

Examples of the negative electrode active material include Nb₂O₅, V₂O₅, TiO₂, In₂O₃, ZnO, SnO₂, NiO, ITO, AZO, GZO, ATO, FTO, and lithium composite oxides such as Li₄Ti₅O₁₂ and Li₂Ti₃O₇. Further, additional examples thereof include metals and alloys such as Li, Al, Si, Si—Mn, Si—Co, Si—Ni, Sn, Zn, Sb, Bi, In, and Au, carbon materials, and materials obtained by intercalation of lithium ions between layers of a carbon material such as LiC₂₄ and LiC₆.

The composite body according to the present disclosure can be favorably produced by, for example, applying the method for producing a solid electrolyte described in the above [2]. More specifically, for example, the composite body can be favorably produced by firing the mixture of the calcined body and the active material described above, that is, by subjecting the mixture to the second heating treatment.

[4] Secondary Battery

Next, a secondary battery to which the present disclosure is applied will be described.

A secondary battery according to the present disclosure includes the solid electrolyte according to the present disclosure as described above, and can be produced by, for example, applying the method for producing a solid electrolyte according to the present disclosure described above.

Such a secondary battery has excellent charge-discharge characteristics.

[4-1] Secondary Battery of First Embodiment

Hereinafter, a secondary battery according to a first embodiment will be described.

FIG. 1 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as the secondary battery of the first embodiment.

As shown in FIG. 1, a lithium-ion battery 100 as the secondary battery includes a positive electrode 10, and a solid electrolyte layer 20 and a negative electrode 30, which are sequentially stacked on the positive electrode 10. The lithium-ion battery further includes a current collector 41 in contact with the positive electrode 10 at an opposite face side of the positive electrode 10 from a face thereof facing the solid electrolyte layer 20, and includes a current collector 42 in contact with the negative electrode 30 at an opposite face side of the negative electrode 30 from a face thereof facing the solid electrolyte layer 20. The positive electrode 10, the solid electrolyte layer 20, and the negative electrode 30 are all constituted by a solid phase, and therefore, the lithium-ion battery 100 is a chargeable and dischargeable all solid-state secondary battery.

The shape of the lithium-ion battery 100 is not particularly limited, and may be, for example, a polygonal disk shape or the like, but is a circular disk shape in the configuration shown in the drawing. The size of the lithium-ion battery 100 is not particularly limited, but for example, the diameter of the lithium-ion battery 100 is, for example, 10 mm or more and 20 mm or less, and the thickness of the lithium-ion battery 100 is, for example, 0.1 mm or more and 1.0 mm or less.

When the lithium-ion battery 100 is small and thin in this manner, together with the fact that it is chargeable and dischargeable and is an all solid-state battery, it can be favorably used as a power supply of a portable information terminal such as a smartphone. The lithium-ion battery 100 may be used for a purpose other than the power supply of a portable information terminal as described later.

Hereinafter, the respective configurations of the lithium-ion battery 100 will be described.

[4-1-1] Solid Electrolyte Layer

The solid electrolyte layer 20 is constituted by a material including the solid electrolyte according to the present disclosure described above.

According to this, the ion conductivity of the solid electrolyte layer 20 becomes excellent. Further, the adhesion of the solid electrolyte layer 20 to the positive electrode 10 or the negative electrode 30 can be made excellent. As a result, the properties and reliability of the lithium-ion battery 100 as a whole can be made particularly excellent.

The solid electrolyte layer 20 may contain a component other than the solid electrolyte according to the present disclosure described above. For example, the solid electrolyte layer 20 may contain another solid electrolyte together with the solid electrolyte according to the present disclosure described above.

However, the content of the solid electrolyte according to the present disclosure in the solid electrolyte layer 20 is preferably 80 mass % or more, more preferably 90 mass % or more, and further more preferably 95 mass % or more.

According to this, the effect of the present disclosure as described above is more remarkably exhibited.

The thickness of the solid electrolyte layer 20 is not particularly limited, but is preferably 0.3 μm or more and 1000 μm or less, and more preferably 0.5 μm or more and 100 μm or less from the viewpoint of charge-discharge rate.

Further, from the viewpoint of preventing a short circuit between the positive electrode 10 and the negative electrode 30 due to a lithium dendritic crystal body deposited at the negative electrode 30 side, a value obtained by dividing the measured weight of the solid electrolyte layer 20 by a value obtained by multiplying the apparent volume of the solid electrolyte layer 20 by the theoretical density of the solid electrolyte material, that is, the sintered density is preferably set to 50% or more, and more preferably set to 90% or more.

As a method for forming the solid electrolyte layer 20, for example, a green sheet method, a press firing method, a cast firing method, or the like is exemplified. A specific example of the method for forming the solid electrolyte layer 20 will be described in detail later. For the purpose of improving the adhesion of the solid electrolyte layer 20 to the positive electrode 10 and the negative electrode 30, or improving the output or battery capacity of the lithium-ion battery 100 by an increase in specific surface area, or the like, for example, a three-dimensional pattern structure such as a dimple, trench, or pillar pattern may be formed at a surface of the solid electrolyte layer 20 to come in contact with the positive electrode 10 or the negative electrode 30.

[4-1-2] Positive Electrode

The positive electrode 10 may be any as long as it is constituted by a positive electrode active material that can repeat electrochemical occlusion and release of lithium ions.

Specifically, as the positive electrode active material constituting the positive electrode 10, for example, a lithium composite oxide which contains at least Li and is constituted by any one or more types of elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, and Cu, or the like can be used. Examples of such a composite oxide include LiCoO₂, LiNiO₂, LiMn₂O₄, Li₂Mn₂O₃, LiCr_(0.5)Mn_(0.5)O₂, LiFePO₄, Li₂FeP₂O₇, LiMnPO₄, LiFeBO₃, Li₃V₂(PO₄)₃, Li₂CuO₂, Li₂FeSiO₄, and Li₂MnSiO₄. Further, as the positive electrode active material constituting the positive electrode 10, for example, a fluoride such as LiFeF₃, a boride complex compound such as LiBH₄ or Li₄BN₃H₁₀, an iodine complex compound such as a polyvinylpyridine-iodine complex, a nonmetallic compound such as sulfur, or the like can also be used.

The positive electrode 10 is preferably formed as a thin film at one surface of the solid electrolyte layer 20 in consideration of an electric conduction property and an ion diffusion distance.

The thickness of the positive electrode 10 formed of the thin film is not particularly limited, but is preferably 0.1 μm or more and 500 μm or less, and more preferably 0.3 μm or more and 100 μm or less.

As a method for forming the positive electrode 10, for example, a vapor phase deposition method such as a vacuum vapor deposition method, a sputtering method, a CVD method, a PLD method, an ALD method, or an aerosol deposition method, a chemical deposition method using a solution such as a sol-gel method or an MOD method, or the like is exemplified. In addition, for example, fine particles of the positive electrode active material are formed into a slurry together with an appropriate binder, followed by squeegeeing or screen printing, thereby forming a coating film, and then, the coating film may be baked onto the surface of the solid electrolyte layer 20 by drying and firing.

[4-1-3] Negative Electrode

The negative electrode 30 may be any as long as it is constituted by a so-called negative electrode active material that repeats electrochemical occlusion and release of lithium ions at a lower potential than the material selected as the positive electrode 10.

Specifically, examples of the negative electrode active material constituting the negative electrode 30 include Nb₂O₅, V₂O₅, TiO₂, In₂O₃, ZnO, SnO₂, NiO, ITO, AZO, GZO, ATO, FTO, and lithium composite oxides such as Li₄Ti₅O₁₂ and Li₂Ti₃O₇. Further, additional examples thereof include metals and alloys such as Li, Al, Si, Si—Mn, Si—Co, Si—Ni, Sn, Zn, Sb, Bi, In, and Au, carbon materials, and materials obtained by intercalation of lithium ions between layers of a carbon material such as LiC₂₄ and LiC₆.

The negative electrode 30 is preferably formed as a thin film at one surface of the solid electrolyte layer 20 in consideration of an electric conduction property and an ion diffusion distance.

The thickness of the negative electrode 30 formed of the thin film is not particularly limited, but is preferably 0.1 μm or more and 500 μm or less, and more preferably 0.3 μm or more and 100 μm or less.

As a method for forming the negative electrode 30, for example, a vapor phase deposition method such as a vacuum vapor deposition method, a sputtering method, a CVD method, a PLD method, an ALD method, or an aerosol deposition method, a chemical deposition method using a solution such as a sol-gel method or an MOD method, or the like is exemplified. In addition, for example, fine particles of the negative electrode active material are formed into a slurry together with an appropriate binder, followed by squeegeeing or screen printing, thereby forming a coating film, and then, the coating film may be baked onto the surface of the solid electrolyte layer 20 by drying and firing.

[4-1-4] Current Collector

The current collectors 41 and 42 are electric conductors provided so as to play a role in transfer of electrons to the positive electrode 10 or the negative electrode 30. As the current collector, generally, a current collector constituted by a material that has a sufficiently small electrical resistance, and that does not substantially change the electric conduction property or the mechanical structure thereof by charging and discharging is used. Specifically, as the constituent material of the current collector 41 of the positive electrode 10, for example, Al, Ti, Pt, Au, or the like is used. Further, as the constituent material of the current collector 42 of the negative electrode 30, for example, Cu or the like is favorably used.

The current collectors 41 and 42 are generally provided so that the contact resistance with the positive electrode 10 and the negative electrode 30 becomes small, respectively. Examples of the shape of each of the current collectors 41 and 42 include a plate shape and a mesh shape.

The thickness of each of the current collectors 41 and 42 is not particularly limited, but is preferably 7 μm or more and 85 μm or less, and more preferably 10 μm or more and 60 μm or less.

In the configuration shown in the drawing, the lithium-ion battery 100 includes a pair of current collectors 41 and 42, however, for example, when a plurality of lithium-ion batteries 100 are used by being stacked and electrically coupled to one another in series, the lithium-ion battery 100 may also be configured to include only the current collector 41 of the current collectors 41 and 42.

The lithium-ion battery 100 may be used for any purpose. Examples of an electronic device to which the lithium-ion battery 100 is applied as a power supply include a personal computer, a digital camera, a cellular phone, a smartphone, a music player, a tablet terminal, a timepiece, a smartwatch, various types of printers such as an inkjet printer, a television, a projector, wearable terminals such as a head-up display, wireless headphones, wireless earphones, smart glasses, and a head-mounted display, a video camera, a videotape recorder, a car navigation device, a drive recorder, a pager, an electronic notebook, an electronic dictionary, an electronic translation machine, an electronic calculator, an electronic gaming device, a toy, a word processor, a work station, a robot, a television telephone, a television monitor for crime prevention, electronic binoculars, a POS terminal, a medical device, a fish finder, various types of measurement devices, a device for mobile terminal base stations, various types of meters for vehicles, railroad cars, airplanes, helicopters, ships, or the like, a flight simulator, and a network server. Further, the lithium-ion battery 100 may be applied to, for example, moving objects such as a car and a ship. More specifically, it can be favorably applied as, for example, a storage battery for electric cars, plug-in hybrid cars, hybrid cars, fuel cell cars, or the like. In addition, it can also be applied to, for example, a power supply for household use, a power supply for industrial use, a storage battery for photovoltaic power generation, or the like.

[4-2] Secondary Battery of Second Embodiment

Next, a secondary battery according to a second embodiment will be described.

FIG. 2 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as the secondary battery of the second embodiment, and FIG. 3 is a schematic cross-sectional view schematically showing a structure of the lithium-ion battery as the secondary battery of the second embodiment.

Hereinafter, the secondary battery according to the second embodiment will be described with reference to these drawings, but different points from the above-mentioned embodiment will be mainly described, and the description of the same matter will be omitted.

As shown in FIG. 2, a lithium-ion battery 100 as the secondary battery of this embodiment includes a positive electrode composite material 210 that functions as a positive electrode, and an electrolyte layer 220 and a negative electrode 30, which are sequentially stacked on the positive electrode composite material 210. The lithium-ion battery further includes a current collector 41 in contact with the positive electrode composite material 210 at an opposite face side of the positive electrode composite material 210 from a face thereof facing the electrolyte layer 220, and includes a current collector 42 in contact with the negative electrode 30 at an opposite face side of the negative electrode 30 from a face thereof facing the electrolyte layer 220.

Hereinafter, the positive electrode composite material 210 and the electrolyte layer 220 which are different from the configuration of the lithium-ion battery 100 according to the above-mentioned embodiment will be described.

[4-2-1] Positive Electrode Composite Material

As shown in FIG. 3, the positive electrode composite material 210 in the lithium-ion battery 100 of this embodiment includes a positive electrode active material 211 in a particulate shape and a solid electrolyte 212. In such a positive electrode composite material 210, the battery reaction rate in the lithium-ion battery 100 can be further increased by increasing an interfacial area where the positive electrode active material 211 in a particulate shape and the solid electrolyte 212 are in contact with each other.

The average particle diameter of the positive electrode active material 211 is not particularly limited, but is preferably 0.1 μm or more and 150 μm or less, and more preferably 0.3 μm or more and 60 μm or less.

According to this, it becomes easy to achieve both an actual capacity density close to the theoretical capacity of the positive electrode active material 211 and a high charge-discharge rate.

Note that in this specification, the average particle diameter refers to a volume-based average particle diameter, and can be determined by, for example, subjecting a dispersion liquid prepared by adding a sample to methanol and dispersing the sample for 3 minutes using an ultrasonic disperser to measurement with a particle size distribution analyzer according to the Coulter counter method (model TA-II, manufactured by Coulter Electronics, Inc.) using an aperture of 50 μm.

The particle size distribution of the positive electrode active material 211 is not particularly limited, and for example, in the particle size distribution having one peak, the half width of the peak can be set to 0.15 μm or more and 19 μm or less. Further, the particle size distribution of the positive electrode active material 211 may have two or more peaks.

In FIG. 3, the shape of the positive electrode active material 211 in a particulate shape is shown as a spherical shape, however, the shape of the positive electrode active material 211 is not limited to the spherical shape, and it can have various shapes, for example, a columnar shape, a plate shape, a scaly shape, a hollow shape, an indefinite shape, and the like, and further, two or more types among these may be mixed.

Examples of the positive electrode active material 211 include the same materials as exemplified as the constituent material of the positive electrode 10 in the above-mentioned first embodiment.

In the positive electrode active material 211, for example, a coating layer may be formed at a surface for the purpose of reducing the interface resistance between the positive electrode active material 211 and the solid electrolyte 212, or improving an electron conduction property, or the like. For example, by forming a thin film of LiNbO₃, Al₂O₃, ZrO₂, Ta₂O₅, or the like at a surface of a particle of the positive electrode active material 211 composed of LiCoO₂, the interface resistance of lithium ion conduction can be further reduced. The thickness of the coating layer is not particularly limited, but is preferably 3 nm or more and 1 μm or less.

In this embodiment, the positive electrode composite material 210 includes the solid electrolyte 212 in addition to the positive electrode active material 211 described above. The solid electrolyte 212 is present so as to fill up a gap between particles of the positive electrode active material 211 or so as to be in contact with, particularly in close contact with the surface of the positive electrode active material 211.

The solid electrolyte 212 is constituted by a material including the solid electrolyte according to the present disclosure described above.

According to this, the ion conductivity of the solid electrolyte 212 becomes particularly excellent. Further, the adhesion of the solid electrolyte 212 to the positive electrode active material 211 or the electrolyte layer 220 becomes excellent. Accordingly, the properties and reliability of the lithium-ion battery 100 as a whole can be made particularly excellent.

When the content of the positive electrode active material 211 in the positive electrode composite material 210 is represented by XA [mass %] and the content of the solid electrolyte 212 in the positive electrode composite material 210 is represented by XS [mass %], it is preferred to satisfy a relationship: 0.1≤XS/XA≤8.3, it is more preferred to satisfy a relationship: 0.3≤XS/XA≤2.8, and it is further more preferred to satisfy a relationship: 0.6≤XS/XA≤1.4.

Further, the positive electrode composite material 210 may include an electric conduction assistant, a binder, or the like other than the positive electrode active material 211 and the solid electrolyte 212.

However, the content of the component other than the positive electrode active material 211 and the solid electrolyte 212 in the positive electrode composite material 210 is preferably 10 mass % or less, more preferably 7 mass % or less, and further more preferably 5 mass % or less.

As the electric conduction assistant, any material may be used as long as it is an electric conductor whose electrochemical interaction can be ignored at a positive electrode reaction potential, and more specifically, for example, a carbon material such as acetylene black, Ketjen black, or a carbon nanotube, a noble metal such as palladium or platinum, an electric conductive oxide such as SnO₂, ZnO, RuO₂, ReO₃, or Ir₂O₃, or the like can be used.

The thickness of the positive electrode composite material 210 is not particularly limited, but is preferably 0.1 μm or more and 500 μm or less, and more preferably 0.3 μm or more and 100 μm or less.

[4-2-2] Electrolyte Layer

The electrolyte layer 220 is preferably constituted by the same material or the same type of material as the solid electrolyte 212 from the viewpoint of an interfacial impedance between the electrolyte layer 220 and the positive electrode composite material 210, but may be constituted by a material different from the solid electrolyte 212. For example, the electrolyte layer 220 contains the solid electrolyte according to the present disclosure described above, but may be constituted by a material having a different formulation from the solid electrolyte 212. Further, the electrolyte layer 220 may be a crystalline material or an amorphous material of another oxide solid electrolyte which is not the solid electrolyte according to the present disclosure, a sulfide solid electrolyte, a nitride solid electrolyte, a halide solid electrolyte, a hydride solid electrolyte, a dry polymer electrolyte, or a quasi-solid electrolyte, or may be constituted by a material in which two or more types selected from these are combined.

When the electrolyte layer 220 is constituted by a material containing the solid electrolyte according to the present disclosure, the content of the solid electrolyte according to the present disclosure in the electrolyte layer 220 is preferably 80 mass % or more, more preferably 90 mass % or more, and further more preferably 95 mass % or more.

According to this, the effect of the present disclosure as described above is more remarkably exhibited.

Examples of a crystalline oxide include Li_(0.35)La_(0.55)TiO₃, Li_(0.2)La_(0.27)NbO₃, and a perovskite-type crystal or a perovskite-like crystal in which the elements constituting a crystal thereof are partially substituted with N, F, Al, Sr, Sc, Nb, Ta, Sb, a lanthanoid element, or the like, Li₇La₃Zr₂O₁₂, Li₅La₃Nb₂O₁₂, Li₅BaLa₂TaO₁₂, and a garnet-type crystal or a garnet-like crystal in which the elements constituting a crystal thereof are partially substituted with N, F, Al, Sr, Sc, Nb, Ta, Sb, a lanthanoid element, or the like, Li_(1.3)Ti_(1.7)Al_(0.3)(PO₄)₃, Li_(1.4)Al_(0.4)Ti_(1.6)(PO₄)₃, Li_(1.4)Al_(0.4)Ti_(1.4)Ge_(0.2)(PO₄)₃, and a NASICON-type crystal in which the elements constituting a crystal thereof are partially substituted with N, F, Al, Sr, Sc, Nb, Ta, Sb, a lanthanoid element, or the like, a LISICON-type crystal such as Li₁₄ZnGe₄O₁₆, and other crystalline materials such as Li_(3.4)V_(0.6)Si_(0.4)O₄, Li_(3.6)V_(0.4)Ge_(0.6)O₄, and Li_(2+x)C_(1-x)B_(x)O₃.

Examples of a crystalline sulfide include Li₁₀GeP₂S₁₂, Li_(9.6)P₃S₁₂, Li_(9.54)Si_(1.74)P_(1.44)S_(11.7)Cl_(0.3), and Li₃PS₄.

Examples of other amorphous materials include Li₂O—TiO₂, La₂O₃—Li₂O—TiO₂, LiNbO₃, LiSO₄, Li₄SiO₄, Li₃PO₄—Li₄SiO₄, Li₄GeO₄—Li₃VO₄, Li₄SiO₄—Li₃VO₄, Li₄GeO₄—Zn₂GeO₂, Li₄SiO₄—LiMoO₄, Li₄SiO₄—Li₄ZrO₄, SiO₂—P₂O₅—Li₂O, SiO₂—P₂O₅—LiCl, Li₂O—LiCl—B₂O₃, LiAlCl₄, LiAlF₄, LiF—Al₂O₃, LiBr—Al₂O₃, Li_(2.88)PO_(3.73)N_(0.14), Li₃N—LiCl, Li₆NBr₃, Li₂S—SiS₂, and Li₂S—SiS₂—P₂S₅.

When the electrolyte layer 220 is constituted by a crystalline material, the crystalline material preferably has a crystal structure such as a cubic crystal having small crystal plane anisotropy in the direction of lithium ion conduction. Further, when the electrolyte layer 220 is constituted by an amorphous material, the anisotropy in lithium ion conduction becomes small. Therefore, the crystalline material and the amorphous material as described above are both preferred as a solid electrolyte constituting the electrolyte layer 220.

The thickness of the electrolyte layer 220 is preferably 0.1 μm or more and 100 μm or less, and more preferably 0.2 μm or more and 10 μm or less. When the thickness of the electrolyte layer 220 is a value within the above range, the internal resistance of the electrolyte layer 220 can be further decreased, and also the occurrence of a short circuit between the positive electrode composite material 210 and the negative electrode 30 can be more effectively prevented.

For the purpose of improving the adhesion between the electrolyte layer 220 and the negative electrode 30, or improving the output or battery capacity of the lithium-ion battery 100 by an increase in specific surface area, or the like, for example, a three-dimensional pattern structure such as a dimple, trench, or pillar pattern may be formed at a surface of the electrolyte layer 220 to come in contact with the negative electrode 30.

[4-3] Secondary Battery of Third Embodiment

Next, a secondary battery according to a third embodiment will be described.

FIG. 4 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as the secondary battery of the third embodiment, and FIG. 5 is a schematic cross-sectional view schematically showing a structure of the lithium-ion battery as the secondary battery of the third embodiment.

Hereinafter, the secondary battery according to the third embodiment will be described with reference to these drawings, but different points from the above-mentioned embodiments will be mainly described, and the description of the same matter will be omitted.

As shown in FIG. 4, a lithium-ion battery 100 as the secondary battery of this embodiment includes a positive electrode 10, and an electrolyte layer 220 and a negative electrode composite material 330 that functions as a negative electrode, which are sequentially stacked on the positive electrode 10. The lithium-ion battery further includes a current collector 41 in contact with the positive electrode 10 at an opposite face side of the positive electrode 10 from a face thereof facing the electrolyte layer 220, and includes a current collector 42 in contact with the negative electrode composite material 330 at an opposite face side of the negative electrode composite material 330 from a face thereof facing the electrolyte layer 220.

Hereinafter, the negative electrode composite material 330 which is different from the configuration of the lithium-ion battery 100 according to the above-mentioned embodiments will be described.

[4-3-1] Negative Electrode Composite Material

As shown in FIG. 5, the negative electrode composite material 330 in the lithium-ion battery 100 of this embodiment includes a negative electrode active material 331 in a particulate shape and a solid electrolyte 212. In such a negative electrode composite material 330, the battery reaction rate in the lithium-ion battery 100 can be further increased by increasing an interfacial area where the negative electrode active material 331 in a particulate shape and the solid electrolyte 212 are in contact with each other.

The average particle diameter of the negative electrode active material 331 is not particularly limited, but is preferably 0.1 μm or more and 150 μm or less, and more preferably 0.3 μm or more and 60 μm or less.

According to this, it becomes easy to achieve both an actual capacity density close to the theoretical capacity of the negative electrode active material 331 and a high charge-discharge rate.

The particle size distribution of the negative electrode active material 331 is not particularly limited, and for example, in the particle size distribution having one peak, the half width of the peak can be set to 0.1 μm or more and 18 μm or less. Further, the particle size distribution of the negative electrode active material 331 may have two or more peaks.

In FIG. 5, the shape of the negative electrode active material 331 in a particulate shape is shown as a spherical shape, however, the shape of the negative electrode active material 331 is not limited to the spherical shape, and it can have various shapes, for example, a columnar shape, a plate shape, a scaly shape, a hollow shape, an indefinite shape, and the like, and further, two or more types among these may be mixed.

Examples of the negative electrode active material 331 include the same materials as exemplified as the constituent material of the negative electrode 30 in the above-mentioned first embodiment.

In this embodiment, the negative electrode composite material 330 includes the solid electrolyte 212 in addition to the negative electrode active material 331 described above. The solid electrolyte 212 is present so as to fill up a gap between particles of the negative electrode active material 331 or so as to be in contact with, particularly in close contact with the surface of the negative electrode active material 331.

The solid electrolyte 212 is constituted by a material including the solid electrolyte according to the present disclosure described above.

According to this, the ion conductivity of the solid electrolyte 212 becomes particularly excellent. Further, the adhesion of the solid electrolyte 212 to the negative electrode active material 331 or the electrolyte layer 220 can be made excellent. Accordingly, the properties and reliability of the lithium-ion battery 100 as a whole can be made particularly excellent.

When the content of the negative electrode active material 331 in the negative electrode composite material 330 is represented by XB [mass %] and the content of the solid electrolyte 212 in the negative electrode composite material 330 is represented by XS [mass %], it is preferred to satisfy a relationship: 0.14≤XS/XB≤26, it is more preferred to satisfy a relationship: 0.44≤XS/XB≤4.1, and it is further more preferred to satisfy a relationship: 0.89≤XS/XB≤2.1.

Further, the negative electrode composite material 330 may include an electric conduction assistant, a binder, or the like other than the negative electrode active material 331 and the solid electrolyte 212.

However, the content of the component other than the negative electrode active material 331 and the solid electrolyte 212 in the negative electrode composite material 330 is preferably 10 mass % or less, more preferably 7 mass % or less, and further more preferably 5 mass % or less.

As the electric conduction assistant, any material may be used as long as it is an electric conductor whose electrochemical interaction can be ignored at a negative electrode reaction potential, and more specifically, for example, a carbon material such as acetylene black, Ketjen black, or a carbon nanotube, a noble metal such as palladium or platinum, an electric conductive oxide such as SnO₂, ZnO, RuO₂, ReO₃, or Ir₂O₃, or the like can be used.

The thickness of the negative electrode composite material 330 is not particularly limited, but is preferably 0.1 μm or more and 500 μm or less, and more preferably 0.3 μm or more and 100 μm or less.

[4-4] Secondary Battery of Fourth Embodiment

Next, a secondary battery according to a fourth embodiment will be described.

FIG. 6 is a schematic perspective view schematically showing a configuration of a lithium-ion battery as the secondary battery of the fourth embodiment, and FIG. 7 is a schematic cross-sectional view schematically showing a structure of the lithium-ion battery as the secondary battery of the fourth embodiment.

Hereinafter, the secondary battery according to the fourth embodiment will be described with reference to these drawings, but different points from the above-mentioned embodiments will be mainly described, and the description of the same matter will be omitted.

As shown in FIG. 6, a lithium-ion battery 100 as the secondary battery of this embodiment includes a positive electrode composite material 210, and a solid electrolyte layer 20 and a negative electrode composite material 330, which are sequentially stacked on the positive electrode composite material 210. The lithium-ion battery further includes a current collector 41 in contact with the positive electrode composite material 210 at an opposite face side of the positive electrode composite material 210 from a face thereof facing the solid electrolyte layer 20, and includes a current collector 42 in contact with the negative electrode composite material 330 at an opposite face side of the negative electrode composite material 330 from a face thereof facing the solid electrolyte layer 20.

It is preferred that the respective portions satisfy the same conditions as described for the respective portions corresponding thereto in the above-mentioned embodiments.

In the first to fourth embodiments, another layer may be provided between layers or at a surface of a layer of the respective layers constituting the lithium-ion battery 100. Examples of such a layer include an adhesive layer, an insulating layer, and a protective layer.

[5] Method for Producing Secondary Battery

Next, a method for producing the above-mentioned secondary battery will be described.

[5-1] Method for Producing Secondary Battery of First Embodiment

Hereinafter, a method for producing the secondary battery according to the first embodiment will be described.

FIG. 8 is a flowchart showing the method for producing the lithium-ion battery as the secondary battery of the first embodiment, FIGS. 9 and 10 are schematic views schematically showing the method for producing the lithium-ion battery as the secondary battery of the first embodiment, and FIG. 11 is a schematic cross-sectional view schematically showing another method for forming a solid electrolyte layer.

As shown in FIG. 8, the method for producing the lithium-ion battery 100 of this embodiment includes Step S1, Step S2, Step S3, and Step S4.

Step S1 is a step of forming the solid electrolyte layer 20. Step S2 is a step of forming the positive electrode 10. Step S3 is a step of forming the negative electrode 30. Step S4 is a step of forming the current collectors 41 and 42.

[5-1-1] Step S1

In the step of forming the solid electrolyte layer 20 of Step S1, the solid electrolyte layer 20 is formed by, for example, a green sheet method using the calcined body according to the present disclosure as described above, that is, a calcined body containing a precursor oxide and an oxoacid compound. More specifically, the solid electrolyte layer 20 can be formed as follows.

That is, first, for example, a solution in which a binder such as polypropylene carbonate is dissolved in a solvent such as 1,4-dioxane is prepared, and the solution and the calcined body according to the present disclosure are mixed, whereby a slurry 20 m is obtained. In the preparation of the slurry 20 m, a dispersant, a diluent, a humectant, or the like may be further used as needed.

Subsequently, by using the slurry 20 m, a solid electrolyte forming sheet 20 s is formed. More specifically, as shown in FIG. 9, for example, by using a fully automatic film applicator 500, the slurry 20 m is applied to a predetermined thickness onto a base material 506 such as a polyethylene terephthalate film, whereby the solid electrolyte forming sheet 20 s is formed. The fully automatic film applicator 500 includes an application roller 501 and a doctor roller 502. A squeegee 503 is provided so as to come in contact with the doctor roller 502 from above. A conveyance roller 504 is provided below the application roller 501 at a position opposite thereto, and a stage 505 on which the base material 506 is placed is conveyed in a fixed direction by inserting the stage 505 between the application roller 501 and the conveyance roller 504. The slurry 20 m is fed to a side where the squeegee 503 is provided between the application roller 501 and the doctor roller 502 disposed with a gap therebetween in the conveyance direction of the stage 505. The slurry 20 m with a predetermined thickness is applied to the surface of the application roller 501 by rotating the application roller 501 and the doctor roller 502 so as to extrude the slurry 20 m downward from the gap. Then, along with this, by rotating the conveyance roller 504, the stage 505 is conveyed so that the base material 506 comes in contact with the application roller 501 to which the slurry 20 m has been applied. By doing this, the slurry 20 m applied to the application roller 501 is transferred in a sheet form to the base material 506, whereby the solid electrolyte forming sheet 20 s is formed.

Thereafter, the solvent is removed from the solid electrolyte forming sheet 20 s formed on the base material 506, and the solid electrolyte forming sheet 20 s is detached from the base material 506 and punched to a predetermined size using a punching die as shown in FIG. 10, whereby a molded material 20 f is formed.

Thereafter, the molded material 20 f is subjected to a heating step at a temperature of 700° C. or higher and 1000° C. or lower, whereby the solid electrolyte layer 20 as a main fired body is obtained. The heating time and atmosphere in the heating step are as described above.

The solid electrolyte forming sheet 20 s with a predetermined thickness may be formed by pressing and extruding the slurry 20 m by the application roller 501 and the doctor roller 502 so that the sintered density of the solid electrolyte layer 20 after firing becomes 90% or more.

[5-1-2] Step S2

After Step S1, the process proceeds to Step S2.

In the step of forming the positive electrode 10 of Step S2, the positive electrode 10 is formed at one face of the solid electrolyte layer 20. More specifically, for example, first, by using a sputtering device, sputtering is performed using LiCoO₂ as a target in an inert gas such as argon gas, whereby a LiCoO₂ layer is formed at a surface of the solid electrolyte layer 20. Thereafter, the LiCoO₂ layer formed on the solid electrolyte layer 20 is fired in an oxidizing atmosphere so as to convert the crystal of the LiCoO₂ layer into a high-temperature phase crystal, whereby the LiCoO₂ layer can be converted into the positive electrode 10. The firing conditions for the LiCoO₂ layer are not particularly limited, but the heating temperature can be set to 400° C. or higher and 600° C. or lower, and the heating time can be set to 1 hour or more and 3 hours or less.

[5-1-3] Step S3

After Step S2, the process proceeds to Step S3.

In the step of forming the negative electrode 30 of Step S3, the negative electrode 30 is formed at the other face of the solid electrolyte layer 20, that is, a face at an opposite side from the face at which the positive electrode 10 is formed. More specifically, for example, by using a vacuum deposition device or the like, the negative electrode 30 can be formed by forming a thin film of metal Li at a face of the solid electrolyte layer 20 at an opposite side from the face at which the positive electrode 10 is formed. The thickness of the negative electrode 30 can be set to, for example, 0.1 μm or more and 500 μm or less.

[5-1-4] Step S4

After Step S3, the process proceeds to Step S4.

In the step of forming the current collectors 41 and 42 of Step S4, the current collector 41 is formed so as to come in contact with the positive electrode 10, and the current collector 42 is formed so as to come in contact with the negative electrode 30. More specifically, for example, an aluminum foil formed into a circular shape by punching or the like is joined to the positive electrode 10 by pressing, whereby the current collector 41 can be formed. Further, for example, a copper foil formed into a circular shape by punching or the like is joined to the negative electrode 30 by pressing, whereby the current collector 42 can be formed. The thickness of each of the current collectors 41 and 42 is not particularly limited, but can be set to, for example, 10 μm or more and 60 μm or less. In this step, only one of the current collectors 41 and 42 may be formed.

The method for forming the solid electrolyte layer 20 is not limited to the green sheet method shown in Step S1. As another method for forming the solid electrolyte layer 20, for example, a method as described below can be adopted. That is, as shown in FIG. 11, the molded material 20 f may be obtained by filling the calcined body according to the present disclosure in a powder form, that is, a calcined body containing a precursor oxide and an oxoacid compound in a pellet die 80, closing the pellet die using a lid 81, and pressing the lid 81 to perform uniaxial press molding. A treatment for the molded material 20 f thereafter can be performed in the same manner as described above. As the pellet die 80, a die including an exhaust port (not shown) can be favorably used.

[5-2] Method for Producing Secondary Battery of Second Embodiment

Next, a method for producing the secondary battery according to the second embodiment will be described.

FIG. 12 is a flowchart showing the method for producing the lithium-ion battery as the secondary battery of the second embodiment, and FIGS. 13 and 14 are schematic views schematically showing the method for producing the lithium-ion battery as the secondary battery of the second embodiment.

Hereinafter, the method for producing the secondary battery according to the second embodiment will be described with reference to these drawings, but different points from the above-mentioned embodiment will be mainly described, and the description of the same matter will be omitted.

As shown in FIG. 12, the method for producing the lithium-ion battery 100 of this embodiment includes Step S11, Step S12, Step S13, and Step S14.

Step S11 is a step of forming the positive electrode composite material 210. Step S12 is a step of forming the electrolyte layer 220. Step S13 is a step of forming the negative electrode 30. Step S14 is a step of forming the current collectors 41 and 42.

[5-2-1] Step S11

In the step of forming the positive electrode composite material 210 of Step S11, the positive electrode composite material 210 is formed.

The positive electrode composite material 210 can be formed, for example, as follows.

That is, first, for example, a slurry 210 m as a mixture of the positive electrode active material 211 such as LiCoO₂, the calcined body according to the present disclosure as described above, that is, a calcined body containing a precursor oxide and an oxoacid compound, a binder such as polypropylene carbonate, and a solvent such as 1,4-dioxane is obtained. In the preparation of the slurry 210 m, a dispersant, a diluent, a humectant, or the like may be further used as needed.

Subsequently, by using the slurry 210 m, a positive electrode composite material forming sheet 210 s is formed. More specifically, as shown in FIG. 13, for example, by using a fully automatic film applicator 500, the slurry 210 m is applied to a predetermined thickness onto a base material 506 such as a polyethylene terephthalate film, whereby the positive electrode composite material forming sheet 210 s is formed.

Thereafter, the solvent is removed from the positive electrode composite material forming sheet 210 s formed on the base material 506, and the positive electrode composite material forming sheet 210 s is detached from the base material 506 and punched to a predetermined size using a punching die as shown in FIG. 14, whereby a molded material 210 f is formed.

Thereafter, the molded material 210 f is subjected to a heating step at a temperature of 700° C. or higher and 1000° C. or lower, whereby the positive electrode composite material 210 including a solid electrolyte is obtained. The heating time and atmosphere in the heating step are as described above.

[5-2-2] Step S12

After Step S11, the process proceeds to Step S12.

In the step of forming the electrolyte layer 220 of Step S12, the electrolyte layer 220 is formed at one face 210 b of the positive electrode composite material 210. More specifically, for example, by using a sputtering device, sputtering is performed using LiCoO₂ as a target in an inert gas such as argon gas, whereby a LiCoO₂ layer is formed at a surface of the positive electrode composite material 210. Thereafter, the LiCoO₂ layer formed on the positive electrode composite material 210 is fired in an oxidizing atmosphere so as to convert the crystal of the LiCoO₂ layer into a high-temperature phase crystal, whereby the LiCoO₂ layer can be converted into the electrolyte layer 220. The firing conditions for the LiCoO₂ layer are not particularly limited, but the heating temperature can be set to 400° C. or higher and 600° C. or lower, and the heating time can be set to 1 hour or more and 3 hours or less.

[5-2-3] Step S13

After Step S12, the process proceeds to Step S13.

In the step of forming the negative electrode 30 of Step S13, the negative electrode 30 is formed at an opposite face side of the electrolyte layer 220 from a face thereof facing the positive electrode composite material 210. More specifically, for example, by using a vacuum deposition device or the like, the negative electrode 30 can be formed by forming a thin film of metal Li at an opposite face side of the electrolyte layer 220 from a face thereof facing the positive electrode composite material 210.

[5-2-4] Step S14

After Step S13, the process proceeds to Step S14.

In the step of forming the current collectors 41 and 42 of Step S14, the current collector 41 is formed so as to come in contact with the other face of the positive electrode composite material 210, that is, a face 210 a at an opposite side from the face 210 b at which the electrolyte layer 220 is formed, and the current collector 42 is formed so as to come in contact with the negative electrode 30.

The methods for forming the positive electrode composite material 210 and the electrolyte layer 220 are not limited to the above-mentioned methods. For example, the positive electrode composite material 210 and the electrolyte layer 220 may be formed as follows. That is, first, a slurry as a mixture of the calcined body according to the present disclosure, that is, a calcined body containing a precursor oxide and an oxoacid compound, a binder, and a solvent is obtained. Then, the obtained slurry is fed to a fully automatic film applicator 500 and applied onto the base material 506, whereby an electrolyte forming sheet is formed. Thereafter, the electrolyte forming sheet and the positive electrode composite material forming sheet 210 s formed in the same manner as described above are pressed in a stacked state and bonded to each other. Thereafter, a stacked sheet obtained by bonding the sheets to each other is punched to form a molded material, and the molded material is fired in an oxidizing atmosphere, whereby a stacked body of the positive electrode composite material 210 and the electrolyte layer 220 may be obtained.

[5-3] Method for Producing Secondary Battery of Third Embodiment

Next, a method for producing the secondary battery according to the third embodiment will be described.

FIG. 15 is a flowchart showing the method for producing the lithium-ion battery as the secondary battery of the third embodiment, and FIGS. 16 and 17 are schematic views schematically showing the method for producing the lithium-ion battery as the secondary battery of the third embodiment.

Hereinafter, the method for producing the secondary battery according to the third embodiment will be described with reference to these drawings, but different points from the above-mentioned embodiments will be mainly described, and the description of the same matter will be omitted.

As shown in FIG. 15, the method for producing the lithium-ion battery 100 of this embodiment includes Step S21, Step S22, Step S23, and Step S24.

Step S21 is a step of forming the negative electrode composite material 330. Step S22 is a step of forming the electrolyte layer 220. Step S23 is a step of forming the positive electrode 10. Step S24 is a step of forming the current collectors 41 and 42.

[5-3-1] Step S21

In the step of forming the negative electrode composite material 330 of Step S21, the negative electrode composite material 330 is formed.

The negative electrode composite material 330 can be formed, for example, as follows.

That is, first, for example, a slurry 330 m as a mixture of the negative electrode active material 331 such as Li₄Ti₅O₁₂, the calcined body according to the present disclosure, that is, a calcined body containing a precursor oxide and an oxoacid compound, a binder such as polypropylene carbonate, and a solvent such as 1,4-dioxane is obtained. In the preparation of the slurry 330 m, a dispersant, a diluent, a humectant, or the like may be further used as needed.

Subsequently, by using the slurry 330 m, a negative electrode composite material forming sheet 330 s is formed. More specifically, as shown in FIG. 16, for example, by using a fully automatic film applicator 500, the slurry 330 m is applied to a predetermined thickness onto a base material 506 such as a polyethylene terephthalate film, whereby the negative electrode composite material forming sheet 330 s is formed.

Thereafter, the solvent is removed from the negative electrode composite material forming sheet 330 s formed on the base material 506, and the negative electrode composite material forming sheet 330 s is detached from the base material 506 and punched to a predetermined size using a punching die as shown in FIG. 17, whereby a molded material 330 f is formed.

Thereafter, the molded material 330 f is subjected to a heating step at a temperature of 700° C. or higher and 1000° C. or lower, whereby the negative electrode composite material 330 including a solid electrolyte is obtained. The heating time and atmosphere in the heating step are as described above.

[5-3-2] Step S22

After Step S21, the process proceeds to Step S22.

In the step of forming the electrolyte layer 220 of Step S22, the electrolyte layer 220 is formed at one face 330 a of the negative electrode composite material 330. More specifically, for example, by using a sputtering device, sputtering is performed using Li_(2.2)C_(0.8)B_(0.2)O₃ which is a solid solution of Li₂CO₃ and Li₃BO₃ as a target in an inert gas such as argon gas, whereby a Li_(2.2)C_(0.8)B_(0.2)O₃ layer is formed at a surface of the negative electrode composite material 330. Thereafter, the Li_(2.2)C_(0.8)B_(0.2)O₃ layer formed on the negative electrode composite material 330 is fired in an oxidizing atmosphere so as to convert the crystal of the Li_(2.2)C_(0.8)B_(0.2)O₃ layer into a high-temperature phase crystal, whereby the Li_(2.2)C_(0.8)B_(0.2)O₃ layer can be converted into the electrolyte layer 220. The firing conditions for the Li_(2.2)C_(0.8)B_(0.2)O₃ layer are not particularly limited, but the heating temperature can be set to 400° C. or higher and 600° C. or lower, and the heating time can be set to 1 hour or more and 3 hours or less.

[5-3-3] Step S23

After Step S22, the process proceeds to Step S23.

In the step of forming the positive electrode 10 of Step S23, the positive electrode 10 is formed at one face 220 a side of the electrolyte layer 220, that is, an opposite face side of the electrolyte layer 220 from a face thereof facing the negative electrode composite material 330. More specifically, for example, first, by using a vacuum deposition device or the like, a LiCoO₂ layer is formed at one face 220 a of the electrolyte layer 220. Thereafter, a stacked body of the electrolyte layer 220 at which the LiCoO₂ layer is formed, and the negative electrode composite material 330 is fired so as to convert the crystal of the LiCoO₂ layer into a high-temperature phase crystal, whereby the LiCoO₂ layer can be converted into the positive electrode 10. The firing conditions for the LiCoO₂ layer are not particularly limited, but the heating temperature can be set to 400° C. or higher and 600° C. or lower, and the heating time can be set to 1 hour or more and 3 hours or less.

[5-3-4] Step S24

After Step S23, the process proceeds to Step S24.

In the step of forming the current collectors 41 and 42 of Step S24, the current collector 41 is formed so as to come in contact with one face 10 a of the positive electrode 10, that is, the face 10 a of the positive electrode 10 at an opposite side from the face at which the electrolyte layer 220 is formed, and the current collector 42 is formed so as to come in contact with the other face of the negative electrode composite material 330, that is, a face 330 b of the negative electrode composite material 330 at an opposite side from the face 330 a at which the electrolyte layer 220 is formed.

The methods for forming the negative electrode composite material 330 and the electrolyte layer 220 are not limited to the above-mentioned methods. For example, the negative electrode composite material 330 and the electrolyte layer 220 may be formed as follows. That is, first, a slurry as a mixture of the calcined body according to the present disclosure, that is, a calcined body containing a precursor oxide and an oxoacid compound, a binder, and a solvent is obtained. Then, the obtained slurry is fed to a fully automatic film applicator 500 and applied onto the base material 506, whereby an electrolyte forming sheet is formed. Thereafter, the electrolyte forming sheet and the negative electrode composite material forming sheet 330 s formed in the same manner as described above are pressed in a stacked state and bonded to each other. Thereafter, a stacked sheet obtained by bonding the sheets to each other is punched to form a molded material, and the molded material is fired in an oxidizing atmosphere, whereby a stacked body of the negative electrode composite material 330 and the electrolyte layer 220 may be obtained.

[5-4] Method for Producing Secondary Battery of Fourth Embodiment

Next, a method for producing the secondary battery according to the fourth embodiment will be described.

FIG. 18 is a flowchart showing the method for producing the lithium-ion battery as the secondary battery of the fourth embodiment, and FIG. 19 is a schematic view schematically showing the method for producing the lithium-ion battery as the secondary battery of the fourth embodiment.

Hereinafter, the method for producing the secondary battery according to the fourth embodiment will be described with reference to these drawings, but different points from the above-mentioned embodiments will be mainly described, and the description of the same matter will be omitted.

As shown in FIG. 18, the method for producing the lithium-ion battery 100 of this embodiment includes Step S31, Step S32, Step S33, Step S34, Step S35, and Step S36.

Step S31 is a step of forming a sheet for forming the positive electrode composite material 210. Step S32 is a step of forming a sheet for forming the negative electrode composite material 330. Step S33 is a step of forming a sheet for forming the solid electrolyte layer 20. Step S34 is a step of forming a molded material 450 f of molding a stacked body of the sheet for forming the positive electrode composite material 210, the sheet for forming the negative electrode composite material 330, and the sheet for forming the solid electrolyte layer 20 into a predetermined shape. Step S35 is a step of firing the molded material 450 f. Step S36 is a step of forming the current collectors 41 and 42.

In the following description, a description will be made by assuming that Step S32 is performed after Step S31, and Step S33 is performed after Step S32, however, the order of Step S31, Step S32, and Step S33 is not limited thereto, and the order of the steps may be changed, or the steps may be concurrently performed.

[5-4-1] Step S31

In the step of forming a sheet for forming the positive electrode composite material 210 of Step S31, a positive electrode composite material forming sheet 210 s that is the sheet for forming the positive electrode composite material 210 is formed.

The positive electrode composite material forming sheet 210 s can be formed, for example, in the same manner as described in the above second embodiment.

The positive electrode composite material forming sheet 210 s obtained in this step is preferably one obtained by removing the solvent from the slurry 210 m used for forming the positive electrode composite material forming sheet 210 s.

[5-4-2] Step S32

After Step S31, the process proceeds to Step S32.

In the step of forming a sheet for forming the negative electrode composite material 330 of Step S32, a negative electrode composite material forming sheet 330 s that is the sheet for forming the negative electrode composite material 330 is formed.

The negative electrode composite material forming sheet 330 s can be formed, for example, in the same manner as described in the above third embodiment.

The negative electrode composite material forming sheet 330 s obtained in this step is preferably one obtained by removing the solvent from the slurry 330 m used for forming the negative electrode composite material forming sheet 330 s.

[5-4-3] Step S33

After Step S32, the process proceeds to Step S33.

In the step of forming a sheet for forming the solid electrolyte layer 20 of Step S33, a solid electrolyte forming sheet 20 s that is the sheet for forming the solid electrolyte layer 20 is formed.

The solid electrolyte forming sheet 20 s can be formed, for example, in the same manner as described in the above first embodiment.

The solid electrolyte forming sheet 20 s obtained in this step is preferably one obtained by removing the solvent from the slurry 20 m used for forming the solid electrolyte forming sheet 20 s.

[5-4-4] Step S34

After Step S33, the process proceeds to Step S34.

In the step of forming the molded material 450 f of Step S34, the positive electrode composite material forming sheet 210 s, the solid electrolyte forming sheet 20 s, and the negative electrode composite material forming sheet 330 s are pressed in a state of being stacked in this order and bonded to one another. Thereafter, as shown in FIG. 19, a stacked sheet obtained by bonding the sheets to one another is punched, whereby the molded material 450 f is obtained.

[5-4-5] Step S35

After Step S34, the process proceeds to Step S35.

In the step of firing the molded material 450 f of Step S35, the molded material 450 f is subjected to a heating step at a temperature of 700° C. or higher and 1000° C. or lower. By doing this, a portion constituted by the positive electrode composite material forming sheet 210 s is converted into the positive electrode composite material 210, a portion constituted by the solid electrolyte forming sheet 20 s is converted into the solid electrolyte layer 20, and a portion constituted by the negative electrode composite material forming sheet 330 s is converted into the negative electrode composite material 330. That is, a fired body of the molded material 450 f is a stacked body of the positive electrode composite material 210, the solid electrolyte layer 20, and the negative electrode composite material 330. The heating time and atmosphere in the heating step are as described above.

[5-4-6] Step S36

After Step S35, the process proceeds to Step S36.

In the step of forming the current collectors 41 and 42 of Step S36, the current collector 41 is formed so as to come in contact with the face 210 a of the positive electrode composite material 210, and the current collector 42 is formed so as to come in contact with the face 330 b of the negative electrode composite material 330.

Hereinabove, preferred embodiments of the present disclosure have been described, however, the present disclosure is not limited thereto.

For example, the method for producing a solid electrolyte may further include another step in addition to the steps as described above.

Further, when the present disclosure is applied to a secondary battery, the configuration of the secondary battery is not limited to those of the above-mentioned embodiments.

For example, when the present disclosure is applied to a secondary battery, the secondary battery is not limited to a lithium-ion battery, and may be, for example, a secondary battery in which a porous separator is provided between a positive electrode composite material and a negative electrode, and the separator is impregnated with an electrolyte solution.

Further, when the present disclosure is applied to a secondary battery, the production method therefor is not limited to those of the above-mentioned embodiments. For example, the order of the steps in the production of the secondary battery may be made different from that in the above-mentioned embodiments.

Further, in the above-mentioned embodiments, a description has been made by assuming that the solid electrolyte according to the present disclosure constitutes a part of a secondary battery, particularly a part of an all-solid-state lithium secondary battery that is an all-solid-state secondary battery, however, the solid electrolyte according to the present disclosure may constitute, for example, a part other than an all-solid-state secondary battery or may constitute a part other than a secondary battery.

EXAMPLES

Next, specific Examples of the present disclosure will be described. Note that in the following description, room temperature refers to 25° C. at 1 atm. Further, a treatment or measurement for which the temperature condition is not particularly specified was performed at 25° C., and a treatment or measurement for which the pressure condition is not particularly specified was performed in a 1 atm environment.

[6] Production of Calcined Body

First, calcined bodies to be used in the production of solid electrolytes of the below-mentioned respective Examples and respective Comparative Examples were produced.

In the preparation of the respective calcined bodies, metal compound solutions described below were used.

[6-1] Preparation of Metal Compound Solutions Used in Production of Calcined Bodies

[6-1-1] Preparation of 2-n-Butoxyethanol Solution of Lithium Nitrate

In a 30-g reagent bottle made of Pyrex (Pyrex: trademark of Corning Incorporated, Pyrex is a registered trademark) equipped with a magnetic stirring bar, 1.3789 g of lithium nitrate with a purity of 99.95% (3N5, manufactured by Kanto Chemical Co., Inc.) and 18.6211 g of 2-n-butoxyethanol (ethylene glycol monobutyl ether) (Cica Special Grade, manufactured by Kanto Chemical Co., Inc.) were weighed. Then, the reagent bottle was placed on a hot plate with a magnetic stirrer function, and lithium nitrate was completely dissolved in 2-n-butoxyethanol while stirring at 170° C. for 1 hour. The resulting solution was gradually cooled to room temperature, whereby a 2-n-butoxyethanol solution of 1 mol/kg lithium nitrate was obtained.

[6-1-2] Preparation of 2-n-Butoxyethanol Solution of Lanthanum Nitrate

In a 30-g reagent bottle made of Pyrex equipped with a magnetic stirring bar, 8.6608 g of lanthanum nitrate hexahydrate (4N, manufactured by Kanto Chemical Co., Inc.) and 11.3392 g of 2-n-butoxyethanol (Cica Special Grade, manufactured by Kanto Chemical Co., Inc.) were weighed. Then, the reagent bottle was placed on a hot plate with a magnetic stirrer function, and lanthanum nitrate hexahydrate was completely dissolved in 2-n-butoxyethanol while stirring at 140° C. for 30 minutes. The resulting solution was gradually cooled to room temperature, whereby a 2-n-butoxyethanol solution of 1 mol/kg lanthanum nitrate hexahydrate was obtained.

[6-1-3] Preparation of 2-n-Butoxyethanol Solution of Bismuth 2-Ethylhexanoate

In a 20-g reagent bottle made of Pyrex equipped with a magnetic stirring bar, 6.3860 g of bismuth 2-ethylhexanoate (manufactured by Alfa Aesar, Inc.) and 3.6140 g of 2-n-butoxyethanol (Cica Special Grade, manufactured by Kanto Chemical Co., Inc.) were weighed. Then, the reagent bottle was placed on a magnetic stirrer, and bismuth 2-ethylhexanoate was completely dissolved in 2-n-butoxyethanol while stirring at room temperature for 30 minutes, whereby a 2-n-butoxyethanol solution of 1 mol/kg bismuth 2-ethylhexanoate was obtained.

[6-1-4] Preparation of 2-n-Butoxyethanol Solution of Zirconium Tetra-n-Butoxide

In a 20-g reagent bottle made of Pyrex equipped with a magnetic stirring bar, 3.8368 g of zirconium tetra-n-butoxide (manufactured by Kojundo Chemical Lab. Co., Ltd.) and 6.1632 g of 2-n-butoxyethanol (Cica Special Grade, manufactured by Kanto Chemical Co., Inc.) were weighed. Then, the reagent bottle was placed on a hot plate with a magnetic stirrer function, and zirconium tetra-n-butoxide was completely dissolved in 2-n-butoxyethanol while stirring at room temperature for 30 minutes, whereby a 2-n-butoxyethanol solution of 1 mol/kg zirconium tetra-n-butoxide was obtained.

[6-1-5] Preparation of 2-n-Butoxyethanol Solution of Tantalum Pentaethoxide

In a 20-g reagent bottle made of Pyrex equipped with a magnetic stirring bar, 4.0626 g of tantalum pentaethoxide (5N, manufactured by Kojundo Chemical Lab. Co., Ltd.) and 5.9374 g of 2-n-butoxyethanol (Cica Special Grade, manufactured by Kanto Chemical Co., Inc.) were weighed. Then, the reagent bottle was placed on a hot plate with a magnetic stirrer function, and tantalum pentaethoxide was completely dissolved in 2-n-butoxyethanol while stirring at room temperature for 30 minutes, whereby a 2-n-butoxyethanol solution of 1 mol/kg tantalum pentaethoxide was obtained.

[6-1-6] Preparation of 2-n-Butoxyethanol Solution of Antimony Tri-n-Butoxide

In a 20-g reagent bottle made of Pyrex equipped with a magnetic stirring bar, 3.4110 g of antimony tri-n-butoxide (manufactured by Wako Pure Chemical Industries, Ltd.) and 6.5890 g of 2-n-butoxyethanol (Cica Special Grade, manufactured by Kanto Chemical Co., Inc.) were weighed. Then, the reagent bottle was placed on a hot plate with a magnetic stirrer function, and antimony tri-n-butoxide was completely dissolved in 2-n-butoxyethanol while stirring at room temperature for 30 minutes, whereby a 2-n-butoxyethanol solution of 1 mol/kg antimony tri-n-butoxide was obtained.

[6-2] Production of Calcined Bodies According to Respective Examples and Comparative Examples

By using the respective metal compound solutions obtained as described above, calcined bodies according to the respective Examples and the respective Comparative Examples were produced as follows.

Example 1

In this Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 2.930 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 0.070 g of a 2-n-butoxyethanol solution of bismuth 2-ethylhexanoate and 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Example 2

In this Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇(La_(2.90)Bi_(0.10))Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 2.900 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 0.100 g of a 2-n-butoxyethanol solution of bismuth 2-ethylhexanoate and 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Example 3

In this Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇(La_(2.80)Bi_(0.20))Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 2.800 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 0.200 g of a 2-n-butoxyethanol solution of bismuth 2-ethylhexanoate and 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Example 4

In this Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇(La_(2.70)Bi_(0.30))Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 2.700 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 0.300 g of a 2-n-butoxyethanol solution of bismuth 2-ethylhexanoate and 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Example 5

In this Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇(La_(2.65)Bi_(0.35))Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 2.650 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 0.350 g of a 2-n-butoxyethanol solution of bismuth 2-ethylhexanoate and 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Comparative Example 1

In this Comparative Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li_(6.3)La₃(Zr_(1.3)Sb_(0.5)Ta_(0.2))O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 7.560 g of a 2-n-butoxyethanol solution of lithium nitrate and 3.000 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Comparative Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 1.300 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, 0.500 g of a 2-n-butoxyethanol solution of antimony tri-n-butoxide, and 0.200 g of a 2-n-butoxyethanol solution of tantalum pentaethoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Comparative Example 2

In this Comparative Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇La₃Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 3.000 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Comparative Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide prepared as described above was weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Comparative Example 3

In this Comparative Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇(La_(2.95)Bi_(0.05))Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 2.950 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Comparative Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 0.050 g of a 2-n-butoxyethanol solution of bismuth 2-ethylhexanoate and 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

Comparative Example 4

In this Comparative Example, a calcined body to be used in the production of a solid electrolyte represented by the formulation: Li₇(La_(2.6)Bi_(0.4))Zr₂O₁₂ was produced as follows.

First, in a reagent bottle made of Pyrex, 9.100 g of a 2-n-butoxyethanol solution of lithium nitrate and 2.600 g of a 2-n-butoxyethanol solution of lanthanum nitrate hexahydrate, each of which was prepared as described above, and 2 mL of 2-n-butoxyethanol as an organic solvent were weighed. A magnetic stirring bar was put therein, and the reagent bottle was placed on a hot plate with a magnetic stirrer function.

Heating and stirring were performed for 30 minutes by setting the set temperature of the hot plate to 160° C. and the rotation speed to 500 rpm.

Subsequently, 2 mL of 2-n-butoxyethanol was added thereto, and heating and stirring were performed again for 30 minutes. When 30 minute-heating and stirring is regarded as a one-time dehydration treatment, in this Comparative Example, a dehydration treatment is regarded as having been performed twice.

After the dehydration treatment, the reagent bottle was covered with a lid and sealed.

Subsequently, stirring was performed by setting the set temperature of the hot plate to 25° C. and the rotation speed to 500 rpm, thereby gradually cooling the reaction mixture to room temperature.

Subsequently, the reagent bottle was transferred to a dry atmosphere, and in the reagent bottle, 0.400 g of a 2-n-butoxyethanol solution of bismuth 2-ethylhexanoate and 2.000 g of a 2-n-butoxyethanol solution of zirconium tetra-n-butoxide, each of which was prepared as described above, were weighed. A magnetic stirring bar was put therein.

Subsequently, stirring was performed at room temperature for 30 minutes by setting the rotation speed of a magnetic stirrer to 500 rpm, whereby a precursor solution was obtained.

Subsequently, the obtained precursor solution was placed in a dish made of titanium having an inner diameter of 50 mm and a height of 20 mm. This dish was placed on a hot plate and heated for 1 hour by setting the set temperature of the hot plate to 160° C., and then heated for 30 minutes by setting the set temperature of the hot plate to 180° C., thereby removing the solvent.

Subsequently, the dish was heated for 30 minutes by setting the set temperature of the hot plate to 360° C., thereby decomposing most of the contained organic components by combustion.

Thereafter, the dish was heated for 1 hour by setting the set temperature of the hot plate to 540° C., thereby burning and decomposing the remaining organic components. Then, the dish was gradually cooled to room temperature on the hot plate, whereby a solid composition as the calcined body was obtained.

[6-3] Production of Solid Electrolyte

Solid electrolytes were produced as follows using the calcined bodies according to the respective Examples and the respective Comparative Examples obtained as described above, respectively.

First, the calcined body was transferred to an agate mortar and sufficiently ground. A 0.150 g portion of the thus obtained ground material of the calcined body was weighed out and placed in a pellet die with an exhaust port having an inner diameter of 10 mm as a molding die, pressed at a pressure of 0.624 kN/mm² for 5 minutes, whereby a calcined body pellet that is a disk-shaped molded material was produced.

Then, the calcined body pellet was placed in a crucible made of magnesium oxide, the crucible was covered with a lid made of magnesium oxide, and then, the pellet was subjected to main firing in an electric muffle furnace FP311 manufactured by Yamato Scientific Co., Ltd. The main firing conditions were set to 900° C. and 8 hours. Subsequently, the electric muffle furnace was gradually cooled to room temperature, and then, the disk-shaped solid electrolyte having a diameter of about 9.5 mm and a thickness of about 600 μm was taken out from the crucible.

The formulation and the crystal phase of each of the solid electrolytes of the respective Examples and the respective Comparative Examples are collectively shown in Table 1. The crystal phase of the solid electrolyte was specified from an X-ray diffraction pattern obtained by the measurement using an X-ray diffractometer X′Pert-PRO manufactured by Koninklijke Philips N.V. In Table 1, a tetragonal crystal structure is denoted by “t”, and a cubic crystal structure is denoted by “c”. Note that the content of the oxoacid compound in each of the solid electrolytes of the respective Examples and the respective Comparative Examples was 10 ppm or less. Further, the content of the solvent in each of the calcined bodies according to the respective Examples and the respective Comparative Examples was 0.1 mass % or less. In addition, when a portion of each of the calcined bodies according to the respective Examples was measured by TG-DTA at a temperature raising rate of 10° C./min, only one exothermic peak was observed in a range of 300° C. or higher and 1,000° C. or lower in all the cases. From the results, it can be said that the calcined bodies according to the respective Examples are constituted by a substantially single crystal phase.

TABLE 1 Value of x in compositional Formulation of Crystal formula (1) solid electrolyte phase Example 1 0.07 Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ c Example 2 0.10 Li₇(La_(2.90)Bi_(0.10))Zr₂O₁₂ c Example 3 0.20 Li₇(La_(2.80)Bi_(0.20))Zr₂O₁₂ c Example 4 0.30 Li₇(La_(2.70)Bi_(0.30))Zr₂O₁₂ c Example 5 0.35 Li₇(La_(2.65)Bi_(0.35))Zr₂O₁₂ c Comparative 0.00 Li_(6.3)La₃(Zr_(1.3)Sb_(0.5)Ta_(0.2))O₁₂ c Example 1 Comparative 0.00 Li₇La₃Zr₂O₁₂ t Example 2 Comparative 0.05 Li₇(La_(2.95)Bi_(0.05))Zr₂O₁₂ c Example 3 Comparative 0.40 Li₇(La_(2.6)Bi_(0.4))Zr₂O₁₂ c Example 4

[6-4] Evaluation of Solid Electrolyte [6-4-1] Evaluation of Total Lithium Ion Conductivity

With respect to each of the disk-shaped solid electrolytes of the respective Examples and the respective Comparative Examples, a circular lithium metal foil having a diameter of 5 mm was pressed against both faces, whereby activated electrodes were formed.

Then, an electrochemical impedance was measured using an AC impedance analyzer Solartron 1260 (manufactured by Solartron Analytical, Inc.), and the total lithium ion conductivity was determined.

The measurement of the electrochemical impedance was performed at an AC amplitude of 10 mV in a frequency range from 10⁷ Hz to 10⁻¹ Hz. The total lithium ion conductivity obtained by the measurement of the electrochemical impedance includes the bulk lithium ion conductivity and the grain boundary lithium ion conductivity in the solid electrolyte.

Further, with respect to the respective Examples and the respective Comparative Examples, disk-shaped solid electrolytes were produced in the same manner as in the above [6-3] except that the main firing conditions were set to 850° C. and 8 hours, and the total lithium ion conductivity was determined in the same manner as described above also for these solid electrolytes.

These results are collectively shown in Table 2.

TABLE 2 Lithium ion conductivity [S · cm⁻¹] Main firing conditions: Main firing conditions: 900° C. × 8 hours 850° C. × 8 hours Bulk lithium Total lithium Bulk lithium Total lithium ion ion ion ion conductivity conductivity conductivity conductivity Example 1 1.3 × 10⁻³ 7.8 × 10⁻⁴ 1.2 × 10⁻³ 7.7 × 10⁻⁴ Example 2 1.5 × 10⁻³ 8.6 × 10⁻⁴ 1.4 × 10⁻³ 8.5 × 10⁻⁴ Example 3 2.1 × 10⁻³ 1.3 × 10⁻³ 2.0 × 10⁻³ 1.1 × 10⁻³ Example 4 2.6 × 10⁻³ 1.6 × 10⁻³ 2.6 × 10⁻³ 1.5 × 10⁻³ Example 5 1.6 × 10⁻³ 1.0 × 10⁻³ 1.5 × 10⁻³ 1.0 × 10⁻³ Comparative — 7.0 × 10⁻⁴ — 1.0 × 10⁻⁵ Example 1 Comparative — 7.8 × 10⁻⁷ — 1.0 × 10⁻⁷ Example 2 Comparative 5.0 × 10⁻⁴ 3.8 × 10⁻⁴ 4.8 × 10⁻⁴ 3.7 × 10⁻⁴ Example 3 Comparative 3.0 × 10⁻⁴ 2.3 × 10⁻⁴ 2.9 × 10⁻⁴ 2.8 × 10⁻⁴ Example 4

As apparent from Table 2, Examples 1 to 5 in which Bi was contained at a predetermined ratio all had an excellent conductivity. On the other hand, Comparative Examples 1 and 2 in which Bi was not contained, Comparative Example 3 in which the content of Bi was too low, and Comparative Example 4 in which the content of Bi was too high all had a poor lithium ion conductivity.

[6-4-2] Evaluation of Potential Window

With respect to each of the disk-shaped solid electrolytes of the respective Examples, a lithium metal foil was bonded to one face and a copper foil was bonded to the other face, and the resultant was used as an electrochemical measurement cell. The CV measurement was performed using an electrochemical measurement device AUTOLAB (manufactured by Metrohm Autolab, Inc.). A reference electrode and a counter electrode were coupled to the lithium metal foil, and also a working electrode was coupled to the copper foil. The potential (−3.06 V vs. SHE) of the lithium metal was set to 0 V, and a response current was measured while sweeping the potential at a rate of 0.04 V/sec in a range from −1 to 5 V.

The CV measurement was performed, and a sweep potential-response current in the second cycle in which the lithium ion concentration distribution is equilibrated was plotted as shown in FIG. 20. As a result of the measurement, two peaks were observed in the redox response current, and a reduction current for reducing and depositing a lithium ion as lithium metal when the potential sweep direction is 0→−1 V, and an oxidation current accompanying the ionization of lithium metal when the potential sweep direction is 0→1 V seemed to correspond to the respective peaks. Although a slight deviation from a potential of 0 V is observed in both currents, it is inferred that this is caused by activation energy accompanying a redox reaction or an overvoltage including an interface resistance with an electrode or an ohmic drop.

On the other hand, the response current other than the redox of lithium metal was less than the detection limit between −1 and 5 V, and therefore, the solid electrolytes of the respective Examples are each considered to be a stable solid electrolyte which conducts only lithium ions at 0 to 4 V (vs. SHE) that is the working potential range of the battery.

Incidentally, it is considered that a peak in the vicinity of No. 2 to No. 3 in the CV curve shown in FIG. 20 represents a reduction deposition current from a lithium ion to lithium metal, and a peak in the vicinity of No. 4 to No. 5 represents an oxidation dissolution current for ionizing and dissolving lithium metal. A peak current attributed to a redox response current of a crystal component itself of each of the solid electrolytes of the respective Examples is not observed. Note that at a potential at which lithium ion lithium metal occurs, a battery operation is not practically performed.

[6-4-3] Evaluation of Raman Scattering Spectrum

With respect to the disk-shaped solid electrolytes of the respective Examples and the respective Comparative

Examples, measurement was performed using a Raman spectrometer S-2000 manufactured by JEOL Ltd., whereby Raman scattering spectra were obtained. From the Raman scattering spectra, a crystal system (a cubic crystal or a tetragonal crystal) was confirmed.

As a result, in Comparative Example 2, the spectrum was split into three peaks in each of a region from 200 cm⁻¹ to 300 cm⁻¹ and a region from 300 cm⁻¹ to 400 cm⁻¹, however, in the respective Examples, degeneracy occurred and a broad mountain-shaped peak was exhibited in all the cases. It is considered that in Comparative Example 2, lithium was fixed to the original site, however, in the respective Examples, lithium could freely move, so that degeneracy occurred in the spectrum. In Comparative Example 2, the crystal system is a tetragonal crystal (t) having a low lithium ion conductivity, and in the respective Examples, the crystal system is a cubic crystal (c) having a remarkably high lithium ion conductivity.

The Raman scattering spectra of the solid electrolytes of Example 1 and Comparative example 2 are representatively shown in FIG. 21.

[7] Production of Solid Electrolyte-Coated Positive Electrode Active Material Powder Example 6

A precursor solution prepared in the same manner as described in the above Example 1, and LiCoO₂ particles as a positive electrode active material for a lithium-ion secondary battery were prepared and mixed at a predetermined ratio, and then, subjected to ultrasonic dispersion for 2 hours at 55° C. under the conditions of an oscillation frequency of 38 kHz and an output of 80 W using an ultrasonic cleaner with a temperature adjusting function, US-1 manufactured by AS ONE Corporation.

Thereafter, the resultant was centrifuged at 10,000 rpm for 3 minutes using a centrifuge, and the supernatant was removed.

The obtained precipitate was transferred to a crucible made of magnesium oxide, the crucible was covered with a lid, and by using an atmosphere controlled furnace, while supplying dry air at a flow rate of 1 L/min, the precipitate was fired at 360° C. for 30 minutes, and thereafter fired at 540° C. for 1 hour, and thereafter fired at 900° C. for 3 hours, and then, cooled to room temperature. By doing this, a solid electrolyte-coated positive electrode active material powder containing many constituent particles in which the LiCoO₂ particles that are base particles were each coated with a coating layer constituted by a garnet-type solid electrolyte represented by Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ was obtained.

Examples 7 and 8

Solid electrolyte-coated positive electrode active material powders were produced in the same manner as in the above Example 6 except that the thickness of the coating layer was changed by adjusting the mixing ratio of the precursor solution and the LiCoO₂ particles.

Example 9

A solid electrolyte-coated positive electrode active material powder was produced in the same manner as in the above Example 8 except that LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ particles were used in place of the LiCoO₂ particles as the positive electrode active material for a lithium-ion secondary battery.

Comparative Example 5

In this Comparative Example, a coating layer was not formed for LiCoO₂ particles as the positive electrode active material for a lithium-ion secondary battery, and an aggregate of the LiCoO₂ particles were directly used as a positive electrode active material powder. In other words, a positive electrode active material powder that is not coated with a solid electrolyte was prepared in place of a solid electrolyte-coated positive electrode active material powder.

Comparative Example 6

By using a sputtering device, a coating layer constituted by LiNbO₃ that is a solid electrolyte was deposited to a thickness of 4 nm at surfaces of LiCoO₂ particles as the positive electrode active material for a lithium-ion secondary battery, whereby a solid electrolyte-coated positive electrode active material powder was prepared.

Comparative Example 7

In this Comparative Example, a coating layer was not formed for LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ particles as the positive electrode active material for a lithium-ion secondary battery, and an aggregate of the LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ particles were directly used as a positive electrode active material powder. In other words, a positive electrode active material powder that is not coated with a solid electrolyte was prepared in place of a solid electrolyte-coated positive electrode active material powder.

In all the solid electrolyte-coated positive electrode active material powders according to Examples 6 to 9 and Comparative Example 6 and the positive electrode active material powders according to Comparative Examples 5 and 7 obtained as described above, the content of the solvent was 0.1 mass % or less, and the content of oxoanions was 100 ppm or less. Further, when reflection electron images were obtained by measurement using a field-emission scanning electron microscope with EDS (manufactured by JEOL Ltd.), none was observed at the surface of the positive electrode active material powder in which a coating layer was not formed.

In the constituent particles of the solid electrolyte-coated positive electrode active material powder in which the coating layer of Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ was formed at the surfaces of the LiCoO₂ particles, a white contrast was observed at the surfaces. As the concentration increased, the white contrast increased. This is considered to be Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ generated from the precursor. From an X-ray diffractometer, only a diffraction line attributed to LiCoO₂ was confirmed in each particle, and therefore, the film thickness of the coating layer is considered to be thin to such an extent that the diffraction intensity derived from Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ is below the lower detection limit. According to the above field-emission scanning electron microscope with EDS (manufactured by JEOL Ltd.), the coating layer was thin, and Bi whose content was low was not detected, however, La and Zr were detected at the surfaces of the LiCoO₂ particles. Based on the compositional ratio of Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂, the compositional ratio of La to Zr is 2.95:1.7, and the content ratio of La to Zr detected by this measurement was 3.1:1 in molar ratio, and therefore, the compositional ratio substantially coincides therewith, so that Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ is considered to be generated. Further, when with respect to the coating layers after the first heating step in the process for producing the solid electrolyte-coated positive electrode active material powders of the above Examples 6 to 9, measurement was performed using TG-DTA at a temperature raising rate of 10° C./min, only one exothermic peak was observed in a range of 300° C. or higher and 1,000° C. or lower in all the cases. From the results, it can be said that in the above Examples 6 to 9, the coating layer after the first heating step is formed from a substantially single crystal phase. In the above Examples 6 to 9, the coating layer of the constituent particle of the finally obtained solid electrolyte-coated positive electrode active material powder was constituted by a solid electrolyte having a garnet-type crystal phase, however, the precursor oxide constituting the coating layer after the first heating step had a pyrochlore-type crystal. In the above Examples 6 to 9, the content of the liquid component contained in the composition after the first heating step was 0.1 mass % or less in all the cases. In addition, in the above Examples 6 to 9, the crystal grain diameter of the oxide contained in the coating layer after the first heating step was 20 nm or more and 160 nm or less in all the cases.

The configurations of the solid electrolyte-coated positive electrode active material powders according to Examples 6 to 9 and Comparative Example 6 and the positive electrode active material powders according to Comparative Examples 5 and 7 are collectively shown in Table 3.

TABLE 3 Base particles Coating layer Average particle Thickness Formulation diameter D [μm] Formulation Crystal phase T [nm] T/D Example 6 LiCoO₂ 7.0 Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ garnet-type  4.7 0.0007 Example 7 LiCoO₂ 7.0 Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ garnet-type 22.5 0.0031 Example 8 LiCoO₂ 7.0 Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ garnet-type 36.1 0.0052 Example 9 LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ 7.0 Li₇(La_(2.93)Bi_(0.07))Zr₂O₁₂ garnet-type 30.0 0.0043 Comparative LiCoO₂ 7.0 — — — — Example 5 Comparative LiCoO₂ 7.0 LiNbO₃ trigonal  3.0 0.0004 Example 6 system Comparative LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ 7.0 — — — — Example 7

[8] Evaluation of Solid Electrolyte-Coated Positive Electrode Active Material Powder

By using each of the solid electrolyte-coated positive electrode active material powders according to Examples 6 to 9 and Comparative Example 6 obtained as described above, electrical measurement cells were produced as follows. Further, in the following description, a case where the solid electrolyte-coated positive electrode active material powders were used will be described, however, also with respect to Comparative Examples 5 and 7, electrical measurement cells were produced in the same manner except that the positive electrode active material powder was used in place of the solid electrolyte-coated positive electrode active material powder.

First, the solid electrolyte-coated positive electrode active material powder was powder-mixed with acetylene black (DENKA BLACK, manufactured by Denka Company Limited) that is an electric conduction assistant, and then, further a n-methylpyrrolidinone solution of 10 mass % polyvinylidene fluoride (manufactured by Sigma-Aldrich Japan) was added thereto, whereby a slurry was obtained. The content ratio of the solid electrolyte-coated positive electrode active material powder, acetylene black, and polyvinylidene fluoride in the obtained slutty was 90:5:5 in mass ratio.

Subsequently, the slurry was applied onto an aluminum foil and dried under vacuum, whereby a positive electrode was formed.

The formed positive electrode was punched into a disk shape with a diameter of 13 mm, and Celgard #2400 (manufactured by Asahi Kasei Corporation) as a separator was overlapped therewith. Then, an organic electrolyte solution containing LiPF₆ as a solute, and also containing ethylene carbonate and diethylene carbonate as nonaqueous solvents was injected, and as a negative electrode, a lithium metal foil manufactured by Honjo Metal Co., Ltd. was enclosed in a CR2032 coil cell, whereby an electrical measurement cell was obtained. As the organic electrolyte solution, LBG-96533 manufactured by Kishida Chemical Co., Ltd. was used.

Thereafter, the obtained electrical measurement cell was coupled to a battery charge-discharge evaluation system HJ1001SD8 manufactured by Hokuto Denko Corporation, and as CCCV charge and CC discharge, 0.2 C: 8 cycles, 0.5 C: 5 cycles, 1 C: 5 cycles, 2 C: 5 cycles, 3 C: 5 cycles, 5 C: 5 cycles, 8 C: 5 cycles, 10 C: 5 cycles, 16 C: 5 cycles, and 0.2 C: 5 cycles were performed. After cycles were repeated at the same C-rate, the charge-discharge characteristics were evaluated by a method of increasing the C-rate. The charge-discharge current at that time was set by calculation using 137 mAh/g as the actual capacity of LiCoO₂ and 160 mAh/g as the actual capacity of NCM523 based on the weight of the positive electrode active material of each cell.

The discharge capacity at 16 C discharge in the fifth cycle is collectively shown in Table 4. It can be said that as this numerical value is larger, the charge-discharge performance at a high load is excellent.

TABLE 4 Discharge capacity at 16C discharge in 5th cycle [mAh] Example 6 107 Example 7 109 Example 8 100 Example 9 39 Comparative 62 Example 5 Comparative 80 Example 6 (however, capacity decreased at low load side) Comparative 19 Example 7

As apparent from Table 4, according to the present disclosure, excellent results were obtained. On the other hand, in Comparative Examples, satisfactory results could not be obtained. More specifically, in comparison of Examples 6 to 8 with Comparative Examples 5 and 6, in which the LiCoO₂ particles were used as the positive electrode active material for a lithium-ion secondary battery, an apparently excellent result was obtained in Examples 6 to 8 as compared with Comparative Examples 5 and 6. In comparison of Example 9 with Comparative Example 7, in which the LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ particles were used as the positive electrode active material for a lithium-ion secondary battery, an apparently excellent result was obtained in Example 9 as compared with Comparative Example 7.

Further, solid electrolyte-coated positive electrode active material powders were produced in the same manner as in the above Examples 6 to 9 except that each of the precursor solutions prepared in the same manner as described in the above Examples 2 to 5 was used in place of the precursor solution prepared in the same manner as described in the above Example 1, and evaluation was performed in the same manner as in the above [8] with respect to the solid electrolyte-coated positive electrode active material powders, similar results as those of the above Examples 6 to 9 were obtained. 

What is claimed is:
 1. A solid electrolyte, represented by the following compositional formula (1): Li₇(La_(3-x)Bi_(x))Zr₂O₁₂  (1) wherein x satisfies 0.05<x<0.40.
 2. A method for producing a solid electrolyte comprising: a mixing step of mixing multiple types of raw materials containing metal elements included in the following compositional formula (1), thereby obtaining a mixture; a first heating step of subjecting the mixture to a first heating treatment thereby forming a calcined body; and a second heating step of subjecting the calcined body to a second heating treatment thereby forming a crystalline solid electrolyte represented by the following compositional formula (1): Li₇(La_(3-x)Bi_(x))Zr₂O₁₂  (1) wherein x satisfies 0.05<x<0.40.
 3. The method for producing a solid electrolyte according to claim 2, wherein a heating temperature in the first heating step is 500° C. or higher and 650° C. or lower.
 4. The method for producing a solid electrolyte according to claim 2, wherein a heating temperature in the second heating step is 800° C. or higher and 1000° C. or lower.
 5. A composite body, comprising: an active material; and the solid electrolyte according to claim 1 that coats a part of a surface of the active material. 